1. First Contact
You'll get this by progressing through the story.


2. I Ain't Afraid of No Ghost
You'll get this by progressing through the story.


3. Give them a Hand
You'll get this by progressing through the story.


4. Free Bird
You'll get this by progressing through the story.


5. What Ghost Up Must Come Down
You'll get this by progressing through the story.


6. Itsy Bitsy
You'll get this by progressing through the story.


7. Flower Power
You'll get this by progressing through the story.


8. Irish He Was Alive
You'll get this by progressing through the story.


9. Ready Corset Ghost!
You'll get this by progressing through the story.


10. Wine Not
You'll get this by progressing through the story.


11. Family Reunion
You'll get this by progressing through the story.


12. Knight Light
You'll get this by progressing through the story.


13. Sixth Sense
You'll get this by progressing through the story.

Note: After a patch, some achievements are no longer missable since it creates a save file before the point of no return. However, you don't have your ghost form by this point, so you will still be unable to obtain some of them.

14. It's a Me, Boo
When you're standing outside of the mansion, switch to ghost Elena and fly to the roof. Enter a window and you should meet a black ghost. Leave the room, and you should get the achievement.
http://steamproxy.net/sharedfiles/filedetails/?id=294795650
15. What Does the Fox Say
When you enter the trophy room, there should be a fox to the left. Look at it (Space), and you'll get the achievement.

http://steamproxy.net/sharedfiles/filedetails/?id=294810667
16. Fantastic Amphibian

After getting the knife, go to the garden, and enter the secret area covered by vines at the end of the maze. Look at the fountain (Space), and you should get the achievement.
http://steamproxy.net/sharedfiles/filedetails/?id=294811258
17. Light Light

After you get the ring from Fleur, go to the observatory and switch to ghost Elena. The logo should be on the side of the stairs.
http://steamproxy.net/sharedfiles/filedetails/?id=294823199
18. This S**t is Bananas
After you go through the wine cellar passage, you should be in an area with a long staircase in the middle of the area. Go down to the ground floor, and the room to the right should be a nursey. In there you should find a picture of a banana on the wall. Look at it (Space), and you should get the achievement.
http://steamproxy.net/sharedfiles/filedetails/?id=294836027
19. BOO-UYA
After you lose your powers and go through the secret entrance in the observatory, you'll be in some sort of basement with spiders crawling on the ceiling. The logo is on a wall here and hard to miss. Look at it (Space), and you should get the achievement.
http://steamproxy.net/sharedfiles/filedetails/?id=294836041
20. Rest Your Soul
After you lose your powers and have gone through the basement with spiders mentioned in 19., you should be in a new area. The first room you're in should have the fireplace. Look at it (Space), and you should get the achievement.
http://steamproxy.net/sharedfiles/filedetails/?id=294836068

Actually, you don't. While the game relies on autosaving and has only one save file, you can circumvent this problem by going to the Windows registry and changing a few things. However, beware that this is likely to mess up your save, so only do this if you're done with the game. (Thanks to uka7777 for pointing out the ability to mess with the registry!)
If you don't feel like messing with the registry (or you're just lazy), just scroll to the bottom.

1. Open Regedit by doing this:

Windows 8.1:
Right-click or tap-and-hold the Start button and then choose Run, type in regedit, push enter.

Windows 7:
Click start, search for regedit, push enter.

Windows XP:
Click start, click Run, type in regedit, push enter.

2. Find [HKEY_CURRENT_USER\Software\Nightlight Interactive\Whispering Willows].

3. Find Player0_Chapter_h1453004736 and change it to, e.g, Chapter 2

4. Find Player0_SceneToLoad_h143699578 and change it to, e.g, Mansion_2

5. Player0_SceneName_h1305074816 and change it to, e.g, Mansion 2

6. Start game and load your save. If it says anything else but what you've changed it to (e.g., Chapter 2, Mansion 2), then something's wrong and you need to recheck. If it works, you should spawn in that area.

Here's a very convenient list of examples of what you need to change if you missed an achievement:

It's a Me, Boo:

• Chapter: Chapter 1
  
• SceneToLoad: Overworld_2
  
• SceneName: Overworld 2

You will spawn outside of the mansion.

What Does the Fox Say:

• Chapter: Chapter 2
  
• SceneToLoad: Mansion_2
  
• SceneName: Mansion 2

You will spawn in the same room as the fox.

Fantastic Amphibian:

• Chapter: Chapter 3
  
• SceneToLoad: Garden_E
  
• SceneName: Garden 5

You will spawn in the same area as the vine-covered entrance.

Light Light:

• Chapter: Chapter 3
  
• SceneToLoad: Observatory
  
• SceneName: Observatory

You will spawn in the observatory.

This S**t is Bananas:

• Chapter: Chapter 3
  
• SceneToLoad: Mansion_3
  
• SceneName: Mansion 3

You will spawn in the area with the long staircase.

BOO-UYA:

• Chapter: Chapter 4
  
• SceneToLoad: Tunnels_2
  
• SceneName: Tunnels 2

You will spawn to the right of the logo in the basement.

Rest Your Soul:

• Chapter: Chapter 4
  
• SceneToLoad: Mansion_4
  
• SceneName: Mansion 4

You will spawn in the same room as the fireplace.


If you don't want to manually edit the registry files, you can download these .reg files:
https://www.dropbox.com/sh/a5vzx55ym5pbvk7/AADe1dxUFAQSBVNb7La8hwOxa

They are named after the achievements, just double click the one you need, start the game and enjoy!