Hello and thanks for using my guide, hopefully it will be helpful to you!
I have collected various information from the existing guides and tested and translated the content to save you the hassle.

The deal is as follows: If you just want a (spoiler-free) quick guide to a 100% run, skip directly to the TL;DR section below.

If you prefer more literacy and are looking for detailed descriptions of specific achievements or are stuck in certain parts of the game, skip to the Achievements/Walkthrough sections, respectively. The true ending is covered as a subsection to the Walkthrough.


Please be aware that there is an error in Act 25 in the English translation that renders it impossible to obtain one achievement unless you manually edit the game file. To fix this issue immediately:

1. Open the file C:\Program Files (x86)\Steam\steamapps\common\LoveDiary\Diary_Data\StreamingAssets\Story\English\1-25 冷战.story in a text editor (I used Notepad++)
2. On line 74, change <Muko> to <木子>.
3. Restart the game and play as normal. You cannot load any game previously saved in Act 25.

Warning: The file contains the script for the two variants of Act 25 and will thus contain minor spoilers if you did not yet reach this variant. Try not to look at anything else while fixing the line if you want to avoid getting spoiled.

Once you've edited the file, make sure to restart the game and be aware that any previously saved games in Act 25 will now be invalid and you have to reach it from Act 24 again.
It seems like this issue has been fixed with the latest patch after I reported it to the developer. I'll leave the instructions just in case.

In case you somehow need to refer to their original (full) names. This guide uses their English names.
Qiang - (郝强)
Chang - (常小青)
Rainy - (雨乔)
Muko - (木子)
Wan - (夏小晚)
Yan - (闫晓君)

Most information in this guide was obtained through the existing Chinese guides
初恋日记成就攻略 for detailed descriptions of all achievements
成就 especially for the tip in the comment section that points to the game data
初（刷）恋（题）日记 部分题目及答案 for a detailed description of the multiple choice questions
初恋日记攻略手册 for a detailed description of unlocking the true ending,
as well as uncovering (through Google) this interesting Baidu post[live.baidu.com] that walks through options with screenshots.
Honourable mention to AStats[astats.astats.nl] for persisting the original achievement names and descriptions when they were replaced by the English versions.

All information has been verified by the author's own playthrough (in the English version).

The correct way to obtain the Act 25 achievement was mostly figured out by myself by examining the game files and some feedback from a WIP guide reader.

This lists everything you have to do during specific parts of the game in order to obtain all achievements and the true ending.

In addition to the Act-specific notes below, every time you have a dialogue option involving Muko or Rainy, save your game beforehand and retry the interaction until you obtain a positive result on the relationship/favourability bar at the top of the screen, or you have tried them all (not answering within the time limit is also an option, and not every interaction has a favourable outcome).
In addition to that, every time you enter 'observe classroom' mode, always select Muko - sometimes it gives a bonus and sometimes not.

In the list below, I am referring to the Act (话) numbers throughout the story. Whenever you are browsing a saved game, you can identify the act numbers below the episode number, following the pattern 第1话 = Act 1, etc.

Act 1: The solution to the nightmare maze is room 1-2-4-8 (as the game tells you).
Act 2: When Chang asks about your confession, tell him about it before time runs out for the dialogue option.
Act 2: The correct answer to the classroom question is C (you can't fail it).
Act 5: The solution to the nightmare maze is room 3-7-5-Office (办公室).
Act 6: Refer to the DIY achievement for the solution to the tape puzzle.
Act 8: During the National Holiday, study for the Olympiad on the first day. The answer is B.
Act 8: During the National Holiday, go for a walk on the second day.
Act 9: Win the game of Gomoku.
Act 9: When asked about your self-study time, choose to reply to Wan before time runs out for the dialogue option.
Act 10: Ace the eyesight measure test mini-game by scoring 5.3.
Act 13: When Chang confronts you about having gone to the hospital, don't deny it (letting time run out is ok as well).
Act 13: During the nightmare maze, select Muko (turn left from the starting position) until the end.
Act 14: Do not fall off the table during the last part of the exam - wait until the clock reaches 11.
Act 20: When pondering who to invite for the 2008 Olympics, select either Muko or Chang for a respective +1 (unless you failed both Chang interactions earlier, feel free to select Muko)
Act 20: During the evening Math study at home, answer A.
Act 21: During the evening English study at home, answer at least 12 questions correctly before time runs out (10 required for immediate achievement, 12 for a later one). The full sequence solution is D D B A A D B C A D B A C C A C
Act 23: When prompted what to do at home, select Homework (over Sleeping). Then answer the following when analyzing the poem: C B
Act 24: Apologize to Muko when the dialogue option appears (before time runs out)
Act 25: Apologize to Muko again when the dialogue option appears (before time runs out). This option should not appear during your first playthrough. Do this on your second playthrough for the final achievement.
Act 28: After buying the gift for Rainy and encountering Muko, do not tell her about the gift.
Act 30: When prompted by Muko, admit to remembering Rainy's birthday, but do not tell her about what you plan to do.
Act 32: When Muko presents her math question to you, the solution is D.

If you succesfully enter Act 34, you have qualified for the true ending.

Act 34: Select 'Tutoring books' as a gift for Muko.
Act 34: After entering the mall, save the game and select the balloon activity and score a perfect 10 out of 10. The achievement should unlock.
Act 34: Then reload the game, instead enter the gift shop and buy the bunny gloves at the bottom right corner of the shop.

Please note that acts 36-42 contain somewhat disturbing/traumatic scenes that might be triggering for some individuals. You have been warned.

Act 41: The solution to the final nightmare maze is 1-Office(办公室)-4-3-8-5-2-7
Act 41: Upon exiting the final nightmare maze, save the game. During the self-conflict following the interaction with Muko, let time run out during the dialogue option, and let the scene play out until you get the achievement, then reload the game and replay while selecting the dialogue option this time. According to reports and personal experience, this is required for the trigger needed for the final achievement.

For the final achievement, replay Act 24 (you can use a previously saved game from your earlier playthrough) and it should trigger an alternate version of Act 25 where you can now apologize a second time as described earlier.

I am using the Steam names for the achievements, so if you are looking for something specific, you can do a Ctrl+F search using the names copied from the global page.
Edit: While I was preparing this guide, the achievements were updated to have English names and descriptions so they should be easier to search/figure out now.

Whenever possible, I will refer to the Act (话) numbers throughout the story. Whenever you are browsing a saved game, you can identify the act numbers below the episode number, following the pattern 第1话 = Act 1, etc.

Unmissable. Obtained during the beginning of Act 1 upon confessing to Rainy and getting rejected.

Unmissable. Obtained upon completing the nightmare maze sequence shortly after the above. (The sequence is room 1-2-4-8 as the dialogue will plainly tell you.)

Unmissable. Obtained upon completing the puzzle in Act 6 when preparing audio samples for the Drama Competition.

Hovering above the buttons will show the translation of each action. Combining (Merge) will appear once you have selected valid combinations of items.

Do not check the description (Info) of the lighter! For some reason, this shows an untranslated segment and then freezes the game. To be safe, you should save your game before starting the puzzle.

Solution:
The repeater is a red herring, you can ignore it! If you check the description, it mentions that the headphone jack is broken, so we cannot connect it to other devices using the items provided

- Cut the broken headphone cables with the knife (combine knife with each set of headphones in turn)
- Use the lighter to expose the wires (combine lighter with each set of cut headphones in turn)
- Use the tape to connect the parts into a makeshift cable (combine tape with both cut headphones)
- Insert the music tape into the walkman (combine music tape with walkman)
- Insert the English tape into the recorder (combine English tape with recorder)
- Connect the walkman to the recorder using the cable (combine these three items)

Missable. Just in case you are reading these chronologically, this achievement requires its first action in Act 8! Scroll down to see the full description of this achievement around Act 32.

Missable. During the week-long National Holiday in Act 8, go for a walk on the second day.

Missable. At the beginning of Act 9, Muko challenges you to a game of Gomoku[en.wikipedia.org] (Five in a Row). Not to be confused with the other common game Go, although it is played on the same board.

Make sure to save beforehand!

The AI is smart enough to counter obvious moves, but if you try to plan a few turns ahead and look for opportunities to set up forced traps on the board, you should eventually beat it.
Remember that each unbroken row of threes forces the opponent to block either end (and a subsequent half-blocked row of fours obviously forces the opponent to block the other end), so you can lead into combos that can't be stopped if you set up the stones right.

Missable. At the end of Act 9, you will have an optional choice regarding your self-study time. Choose to reply to Wan to unlock this achievement.

Missable. At the beginning of Act 10, for the physical examination test, your eyesight will be measured as a mini-game. Use the directional keys to indicate which way each symbol is facing - hesitating for more than a second will count as a loss. Score a perfect 5.3 to obtain the achievement. Obviously, it is a good idea to save right before the test.

Missable. At the end of Act 13, another nightmare sequence occurs in which you simply have to select the left corridor and follow Muko to the end.

Missable. In Act 14, during the exam, you'll enter free movement for the last few minutes. You can explore the table, but don't fall off, and stall until the clock hits 11 (you can see it in the background although your vision will gradually fade). If you fall off the table, reload and try again.

Unmissable. In Act 20, shortly after pondering who to invite for the 2008 Olympics (the selection here does not matter, although the obvious choice will net you dependence points), the protagonist will ask Muko in person and the achievement should then trigger.

Unmissable. Received in Act 21 during the shopping trip.

Missable. At the evening of Act 21, get at least 10 questions in English correct during the timed sequence, although don't rush through it all too quickly as it might bug the achievement. The correct sequence is D D B A A D B C A D B A C C A C (although if you are reading this guide, you probably aren't struggling much with English in the first place).
Note that you need 12+ questions correct in order to qualify for the Scholar-lord (真*学霸) achievement further down, so try to get more than that to be on the safe side!

Missable. At the evening of Act 23, choose to do Homework and get both questions correct during the timed sequence. In the English version, apparently the translation is quite broken and the choices seem to have shifted positions or even missing. The solution that unlocked the achievement for me was C B.
Note that this is a requirement for the latter achievement Scholar-lord (真*学霸) further down.

Missable. Just in case you are reading these chronologically and wondering why this isn't mentioned - this achievement requires its first action in Act 24, however based on examining the game code, you are likely unable to unlock this at this time of reading. Scroll down to the entry at the end of the achievements list for details.

Missable. After buying the birthday gift for Rainy in Act 28 and encountering Muko, choose not to tell her what you bought. During Act 29, the choice to ask Muko out ('Date her') does not matter for this achievement (but is recommended for the ending). Muko will probe Qiang and he will automatically bluff that he does not remember Rainy's birthday.
In Act 30, when Muko asks directly, answer Yes the first time (to remembering Rainy's birthday), then 'Don't tell' when probed further about any plans. After secretly delivering the gift and being confronted again by Yan (the dialogue option here shouldn't matter), the achievement should unlock upon the next scene change.

Missable. You need to complete three mathematical questions throughout various stages of the game.
1. During the National Holiday in Act 8, study for the Olympiad on the first day. For this problem, the correct solution is B.
2. During the mandatory home study scene in Act 20, the solution is A.
3. During the mandatory question from Muko in Act 32, the solution is D.
The achievement unlocks instantly upon selecting the correct answer in Act 32, unlike the circles achievement below.
Note that this is a requirement for the latter achievement Scholar-lord (真*学霸) further down.

Missable. Succesfully completing the question in Act 32 (see above) also unlocks this achievement, but only after Qiang finishes his explanation (while the other achievement will unlock immediately).

Missable. Succesfully completing 12+ English questions correctly (see Language experts (英语达人) achievement further above) as well as acing the poem (see Literary champion (古典文学) achievement further above) as well as completing all the math questions (see Mathematical genius (数理天才) achievement further above) will net you this combination achievement right after the Two circles (两圆相交) achievement above.

The following achievements require passage into the true ending (Act 34+). Refer to the walkthrough if you need help with that part.

Missable. In Act 34, after obtaining money from dad, select 'Tutoring books' as a gift option for Muko. Achievement obtained after the gift has been delivered a little later. Surprisingly, none of the options affect anything, so no reloading is needed unless you personally feel like being a little more romantic :P

Easter egg: If you choose 'Euphemistic' when asking dad for money, then tell him about the chess game from an earlier act, you will be able to learn the solution from him and beat the guy on the street later on for even more money. Enjoy feeling rich!

Missable. In Act 34, after entering the mall, select balloon shooting as an activity and score a perfect 10 out of 10. Nothing to say except practice makes perfect, keep trying!

Afterwards, make sure to reload and enter the gift shop instead so you can prepare for the Act 36 achievement.

In Act 34, make sure to visit the gift shop and buy the bunny gloves in the lower right corner for Muko.
When reaching school in Act 36, she will pull them out to show again, and the achievement will trigger at that point, along with a relationship boost.

Note that acts 36-42 contain somewhat disturbing/traumatic scenes that might be triggering for some individuals. You have been warned.

Unmissable. By the end of Act 36.

Unmissable. Upon exiting the nightmare maze of Act 41.

Missable. During the self-interaction by Muko after exiting the nightmare maze at the end of Act 41, save the game and when a dialogue selection appears, let time run out without selecting anything.

Afterwards, reload the game and intervene when the dialogue option appears before continuing the playthrough. Due to reports and personal experience, I have reason to believe this is required for triggering the correct ending.

Unmissable. Entering the final examination room in Act 42.

Missable. You need to apologize to Muko twice during Act 24 and Act 25, respectively. Several sources are unclear on how to unlock the Act 25 scene, but examining the game code shows that the required trigger, which I have personally verified, is to clear Episode 1 at least once, in other words, you need a full playthrough of the game beforehand (most likely including the true ending). Then you can simply reload a saved game from an earlier playthrough (e.g. in Act 24) if you want (or just play through the whole thing again). If done correctly, a chapter selection option will occur when starting a new game, and a bonus scene should trigger in Act 20 as well. I have verified that this method works by finishing Act 41 using the "non-a**hole" choice (clicking the dialogue option instead of skipping it) to make sure this triggers correctly, and strongly recommend that you do the same.

In addition to the above, you also need to manually fix a bug in the English translation of the game for the specific scene in question to appear during Act 25. See the 'Read me first' section at the beginning of this guide for details.
This seems to have been fixed with the latest patch (if not, the instructions are still there).

The TL;DR section in the Introduction can be used as a one-size-fits-all quick walkthrough.

Most of the story is kinetic, so you will only need an actual guide for the nightmare maze scenes which are described in the section below.

For the various puzzles and multiple choice questions scattered across the game, please refer to the achievements section where they are all described under their respective achievements:
DIY
Mathematical genius (数理天才)
Language experts (英语达人)
Literary champion (古典文学)
Other than these, there is a minor test question near the beginning of the game that you can't fail (the game will correct you if you are wrong). The correct answer to this question is C. Perish together.

Other than this, you may want to be constantly aware of the true ending, which is determined at the end of Act 33. This is described in the second section below. If you are hunting for achievements, you will need to enter the true ending either way as several of them are behind that part.

Use the dialogue cues to determine the correct order to visit each room. The sequence will reset if broken.

First (Act 1): 1-2-4-8
Second (Act 5): 3-7-5-Office(办公室)
Third (Act 13): None
Fourth (Act 23): None
Last (Act 41): 1-Office(办公室)-4-3-8-5-2-7

To obtain the true ending, three conditions using hidden parameters must be met by the end of Act 33:
1. Perseverance (坚持) <> 0
2. Friendship (友情) >= 1
3. Muko Favourability (木子好感度) >= 15

Perseverance reflects personal psyche and the willingness to succeed. The following interactions give you the chance to obtain the value needed:
- During Act 14, when taking the exam, do not fall off the table but follow the instructions for the achievement Desperate exam (绝望的考场) and remain until time runs out. This gives a +1
- During Act 21, during the timed English sequence, get at least 7 questions correct for a +1
- During Act 23, when choosing what to do at home, select Homework over Sleeping for a +1

Friendship reflects the relationship with Chang (your former deskmate and best friend). The following interactions give you the chance to obtain the value needed:
- During Act 2, when asked about the confession to Rainy, be honest to him about it. Apparently, failing to respond in time WILL NOT count as a +1 (even though Qiang ends up telling it anyway).
- During Act 13, when confronted about going to the hospital, admit to having gone there (even though Qiang will still lie about the reason for going). Apparently, failing to respond in time WILL still count as a +1
- During Act 20, when considering who to invite for the 2008 Olympics, choose Chang (however, you give up a +1 with Muko for this)

I was able to verify the above from looking directly in the game files, so they should be accurate even though they don't reflect exactly what is mentioned in other guides.

Favourability reflects the relationship with Muko, and is raised through a lot of different interactions throughout the game. Fortunately, this progress is constantly visible as a bar at the top of your screen and you can hover the mouse to check it anytime. For this reason I will not attempt to list every interaction, just save often and keep trying every time you get a choice. Keep in mind, however, that not all interactions have a chance to increase favour, so if you tried everything and nothing worked, just move on.
As a rule of thumb, you need the bar to be at least half full by the time you end Act 33. As another rule of thumb, whenever you enter 'observe classroom' mode (with the option to click on circles reflecting different characters) always select Muko as it will often trigger a +1, while the others never do anything important.

Thank you for reading my guide, feel free to leave comments (including corrections) below!
I will try to update for future episodes as well.