Just keeping it super simple, this guide forgoes achievements and flowcharts with the specific purpose of taking you from the beginning to the secret ending with options for all the alternate endings possible in a single run. I've been pretty vague with the text where the alternate endings happen, but the moments where these alternate endings line up are pretty obvious. The emails replies are the most important part and are the keys to the secret ending.

SAVE!

When Kurisu emails you asking "Did you write it?" reply with "scientist" then later "bad scientist," "do better," "outsider," "disgrace" and "why am I still here?" to her subsequent mails.

(note: one ending can't be reached once you do this)


Kurisu will email you about "plans." Reply with "traveling expenses" and read her next message (titled "Idiot!") to which you don't need to reply.


SAVE!

"Now I just need to send it"
Alt ending 1: don't send

send to continue


SAVE!

Kurisu will email you about "The key." Reply "I don't have a key" then "security" to her next message, then "reality" to the message after that, then read the final reply.

"I'm sorry, _____. I lied to you"
Alt ending 2: don't send

send to continue


SAVE!

Kurisu will email you (Explain yourself). Read it, then read her next message later in the chapter (From an unpopular girl).

"Will you... remember me?"
Alt ending 3: don't send
send to continue


Kurisu will email you with the subject "Contact me, idiot!" Reply with "worried" and read her next message.

(save if you want)
'I take one last look at _____'...
interesting path: don't send (not a separate ending, but worth looking at imo)


Kurisu will email you "Where are you now"
Secret ending: reply "where you are"
Alt ending 4: reply anything else/don't reply

Alt ending 5: Get to this point without any of the previous messages with Kurisu (i.e. replay the game from chapter 4)