I will not list achievements that are story-related and unmissable here.

Most of the game can be done in different orders, but I'll still try to list them in the order that you should pay attention to not missing the achievements.

I'll provide hints as well, and the solutions will be shown in spoiler tags.

If you're looking for a beautiful guide, sorry, but my main goal here is to make sure people can play as freely as possible without having to worry about missing achievements.

Note that there are 58 achievements in this game, 28 of which are missable.

A few things you should know...

• Do NOT skip any cutscenes!
  
• Every time you get a new item after you unlock Hoagie, select the item from the inventory and use it on the portrait of another character to send the item to them. Do that for all items obtained prior to unlocking Hoagie as well. Reportedly, the items have to be sent that way, instead of the way you first learn about.
  
• Note that most of the time the description of the achievement will give it away, so be warned. Images also provide strong hints, sometimes just as obvious as the descriptions, but usually a bit less obvious. Achievement names provide vague hints in case you want a challenge.
  
• If you by any chance get a diamond and are NOT moved to yesterday, load the game. Getting the diamond MUST BE the last thing you do before going to yesterday, or else you'll be locked out of an achievement due to a bug.

For people who want a more simplistic checklist of what they should be wary of doing, I'll conveniently list every action that would lock achievements here:

• Skipping cutscenes
  
• Hoagie gives the chattering teeth to an NPC
  
• Hoagie acquires the quill
  
• Hoagie gives the quill to Red Edison
  
• Hoagie powers up his Chron-o-John
  
• Bernard acquires the stamp album
  
• Bernard gets the contract signed by Dr. Fred
  
• Bernard acquires the diamond
  
• Laverne uses an item in the kennel when the switch is off: cat
  
• Laverne uses an item during the human contest: dentures
  
• Laverne uses the hamster in the basement

The list is put together and sorted alphabetically just for your convenience, so you can check against what you have unlocked already:

• ...Take on the Words!
  
• Alas, Our Lazy Speechwriters Didn't Cover This
  
• And Then the Universe Explodes
  
• And Then the Universe Implodes
  
• Are You My Mummy?
  
• Can't Win and Chew Gum at the Same Time
  
• Chef
  
• Dude, It's Like, Cannibalism
  
• Energy Conscious
  
• ESRB Violation
  
• Games History Major
  
• I'm Sure He'd Never Notice
  
• Master Plumber
  
• Mental Patience
  
• Missing Pink
  
• Most Annoying Guest Ever
  
• Music Appreciation Award
  
• Now You're Pushing It
  
• OCD
  
• Olive Branch
  
• Rap Sheet: Forger
  
• Rap Sheet: Fraud
  
• That Joke Never Gets Old
  
• That Only Works With Alligators
  
• That Should Have Worked
  
• Voiding the Warranty
  
• We're All Brothers
  
• WWSCD?
  
• You Call That a Hint?

This section is divided into further sections related to certain actions that lock achievements.


This achievement is also listed under Laverne's section, since there are two actions that can prevent you from unlocking this:

• Hoagie lock: Give the chattering teeth to an NPC.
  
• Laverne lock: Use an item during the human contest: dentures

< image
Can't Win and Chew Gum at the Same Time
Description: Laverne tries the chattering teeth on her human during the human show

Hint: From most difficult to most obvious hint -- Achievement name | Image | Description

----------------------------------------------------------------------------------------------------------------------------------

< image
WWSCD?
Description: Climb through the chimney with all three kids

Hint: Note that Lavern's lock listed above doesn't apply to this achievement. From most difficult to most obvious hint -- Achievement name | Image | Description | Solution: You'll have to climb the chimney with Bernard and Laverne at some point in the game naturally, so you only have to worry about Hoagie, who is not required to climb the chimney, and won't be able to after you light the fireplace up. This will unlock after all three characters have climbed the chimney.


Missing point for this section: After you get the quill, you've lost your chance. The window for this achievement is very short, and it should be done when you're about to get the quill.

< image
Alas, Our Lazy Speechwriters Didn't Cover This
Description: Talk to Washington, Jefferson, or Hancock while they're outside

Hint: From most difficult to most obvious hint -- Achievement name | Image | Description

Warning: Giving the quill to the right NPC afterwards will block you from unlocking an achievement as Bernard, so make sure you check his section.


Missing point for this section: After you power Hoagie's Chron-o-John by doing a certain action (connecting the powered battery to the plug), you've lost your chance. The window for this achievement is very short, and it should be done when you're about to power up the Chron-o-John.

< image
That Should Have Worked
Description: Hoagie uses battery with Chron-o-John instead of plug

Hint: From most difficult to most obvious hint -- Achievement name | Image | Description

This section is divided into further sections related to certain actions that lock achievements.


Missing point for this section: After you get the stamp album, you've lost your chance.

< image
I'm Sure He'd Never Notice
Description: Try to swap the textbook with the stamp album

Hint: From most difficult to most obvious hint -- Achievement name | Image | Description


Missing point for this section: After you get the contract signed by Dr. Fred, you've lost your chance.
Missing point for this section: After Hoagie gives the quill to Red Edison, you've lost your chance for the 2nd achievement listed here.

< image
Rap Sheet: Fraud
Description: Try to alter the contract with Booboo-B-Gone

Hint: From most difficult to most obvious hint -- Achievement name | Image | Requirements: item combination | Description

----------------------------------------------------------------------------------------------------------------------------------

< image
Rap Sheet: Forger
Description: Try to sign the contract yourself with the quill pen

Hint: From most difficult to most obvious hint -- Achievement name | Requirements: item combination | Image | Description

This section is divided into further sections related to certain actions that lock achievements.


This achievement is also listed under Hoagie's section, since there are two actions that can prevent you from unlocking this:

• Hoagie lock: Give the item to an NPC.
  
• Laverne lock: Use an item during the human contest: dentures

< image
Can't Win and Chew Gum at the Same Time
Description: Laverne tries the chattering teeth on her human during the human show

Hint: From most difficult to most obvious hint -- Achievement name | Image | Description


Missing point for this section: After you use an item while the switch is off (cat), you've lost your chance.

< image
ESRB Violation
Description: Laverne thinks about eating the cat

Hint: From most difficult to most obvious hint -- Achievement name | Description | Image | Solution: Combine the fork with the cat in your inventory


Missing point for this section: After you use the hamster in the basement, you've lost your chance.

< image
That Only Works With Alligators
Description: Try to flush the hamster

Hint: You most likely tried to do this before getting to this point in the game. From most difficult to most obvious hint -- Achievement name | Image | Description

< image
Now You're Pushing It
Description: Try to microwave the hamster again after he's thawed out

Hint: From most difficult to most obvious hint -- Achievement name | Image | Description

These achievements must be done any time before you go back to yesterday at the end of the game, in any order. There are three requirements for you to go to yesterday that I'll list below, but be warned that it contains spoilers. A safe approach would be to save often, and load if you end up moving to yesterday accidentally:

1. Repair Hoagie's Chron-o-John
   
2. Repair Laverne's Chron-o-John
   
3. Acquired a diamond (only do this after you've done the other two!!!)

You shouldn't worry about these achievements until right before the last chance. You'll probably get most of them on your own.


< image
Master Plumber
Flush 50 objects through time

Hint: I've warned you about this achievement at the beginning. Reportedly, the items have to be sent through the inventory, instead of the way you first learn how to do it. Some people also say you can do this with the same item multiple times, and it doesn't have to be 50 unique items. If the latter doesn't work and you've run out of items, you could try sending the same items across all 3 characters as well. Unfortunately, I can't check these claims.
Update: The user Rhadamante confirmed this information by sending the same item between 2 characters and unlocking this achievement.

----------------------------------------------------------------------------------------------------------------------------------

< image
Dude, It's Like, Cannibalism
Description: Hoagie eats a Hoagie

Hint: This achievement requires you to do nothing. Literally, with a specific character. Hoagie. Until you get the correct idle animation.

----------------------------------------------------------------------------------------------------------------------------------

< image
Are You My Mummy?
Description: Talk to Dead Cousin Ted with all three kids

Hint: From most difficult to most obvious hint -- Achievement name | Image | Character: All three | Description | Solution: There's a mummy of Dead Cousin Ted in each timeline. Talk to all three.

----------------------------------------------------------------------------------------------------------------------------------

< image
That Joke Never Gets Old
Description: Squirt five different people with disappearing ink

Hint: From most difficult to most obvious hint -- Achievement name | Image | Character: Any or multiple | Description

----------------------------------------------------------------------------------------------------------------------------------

< image
Missing Pink
Description: Try to mix the red paint and the Booboo-B-Gone

Hint: From most difficult to most obvious hint -- Achievement name | Character: Any | Requirements: item combination | Image | Item used: Red paint... | Description

----------------------------------------------------------------------------------------------------------------------------------

< image
And Then the Universe Explodes
Description: Try to mix the decaf and caffeinated coffees

Hint: From most difficult to most obvious hint -- Achievement name | Character: Any | Requirements: item combination | Image | Item used: Coffee... | Description

----------------------------------------------------------------------------------------------------------------------------------

< image
And Then the Universe Implodes
Description: Try to mix the disappearing ink with Booboo-B-Gone

Hint: From most difficult to most obvious hint -- Achievement name | Character: Any | Requirements: item combination | Item used: Disappearing ink... | Image | Description

----------------------------------------------------------------------------------------------------------------------------------

< image
Voiding the Warranty
Description: Try to put something metal in the microwave

Hint: From most difficult to most obvious hint -- Achievement name | Image | Character: Bernard or Laverne | Description | Solution: Use a fork in the microwave, for example. Maybe some other items like the scalpel work, too.

----------------------------------------------------------------------------------------------------------------------------------

< image
Most Annoying Guest Ever
Description: Ring the front desk bell 100 times

Hint: From most difficult to most obvious hint -- Image | Character: Bernard | Achievement name | Description | Solution: It's annoying to unlock, too. Be patient, and use the desk bell all those 100 times.

----------------------------------------------------------------------------------------------------------------------------------

< image
Energy Conscious
Description: Turn off the television

Hint: From most difficult to most obvious hint -- Achievement name | Image | Character: Bernard | Description | Solution: This is the TV on the 2nd floor where you watch the commercial to get the phone number. Seems to work only on the "W" room.

----------------------------------------------------------------------------------------------------------------------------------

< image
You Call That a Hint?
Description: Try to wash the dirty car with the brush

Hint: From most difficult to most obvious hint -- Achievement name | Character: Bernard | Image | Item used: Brush | Description

< image
We're All Brothers
Description: Ask the IRS agents if they're related

Hint: From most difficult to most obvious hint -- Achievement name | Character: Bernard | Location: Attic | Available since: Dr. Fred gets arrested | Image | Description

----------------------------------------------------------------------------------------------------------------------------------

< image
Olive Branch
Description: Return the stamp album to Weird Ed

Hint: From most difficult to most obvious hint -- Achievement name | Image | Character: Bernard | Item: Stamp album | Description

----------------------------------------------------------------------------------------------------------------------------------

< image
Games History Major
Description: Play Maniac Mansion

Hint: From most difficult to most obvious hint -- Achievement name | Image | Character: Bernard | Description | Location: Weird Ed's room | Solution Use the computer and start playing Maniac Mansion

----------------------------------------------------------------------------------------------------------------------------------

< image
Chef
Description: Cook the hamster in Maniac Mansion (Don't try this at home, kids!)

Hint: Starts from the achievement right above this one. The description will give you a general guideline, but I'll provide a walkthrough below in case you need it:

1. Characters to pick: Syd and Razor
   
2. As Dave, walk left, pull doormat, pick up key, use key on door, enter.
   
3. Open and enter the first door, walk to the right until you get captured.
   
4. Select New Kid and pick Syd.
   
5. Follow the same path Dave took, but keep going to the right until you find the pantry. Pick up fruit drinks from the shelves then go back to the main hall with the stairs, and go up.
   
6. Go left, open and enter the door, pick wax fruits, and leave the room.
   
7. Go through big metal doors, go all the way right, climb the stairs.
   
8. Might as well save now. Give FIRST the wax fruits to the tentacle, then the drinks. If you mess up the order, load the game.
   
9. Take the stairs at the left, then move all the way to the right and enter the fourth door.
   
10. Switch to Razor. Walk to the front door. Save!!!
    
11. Use doorbell (it's on the door).
    
12. Switch to Syd. Wait a few seconds. Leave the room (if you're caught, load the game), take the next door to the left of where you where, grab the hamster inside, then run back to the fourth door.
    
13. If you made it back safely inside the fourth room again, SAVE! Wait a few more seconds, and leave. If Ed's back, his door will be closed again. Then you're good to go.
    
14. Go back all the way back to the kitchen. Open the microwave oven. Use hamster with microwave oven. Close the microwave oven. Turn on the microwave oven.

----------------------------------------------------------------------------------------------------------------------------------

< image
Music Appreciation Award
Description: Reach yesterday without turning the stereo off

Hint: The description is a bit off, but provides a very obvious hint. Right before you use the telephone in order to finally get the diamond and go back to yesterday, make sure the stereo on the 2nd floor is turned on. Remember that warning from the "Before you start" section? This achievement will not unlock if you don't finish "today" with Bernard last. Note that the stereo doesn't have to be on during your entire playthrough, it just has to be that way before doing the final action that triggers moving to yesterday.

< image
Mental Patience
Play the whole game without skipping a cutscene

----------------------------------------------------------------------------------------------------------------------------------

< image
...Take on the Words!
Sit through the end credits

----------------------------------------------------------------------------------------------------------------------------------

< image
OCD
Get all other achievements in the game

I hope you enjoyed this guide! I personally love spoiler-free guides that help me get all achievements without ruining the game for me.

If you like this guide, don't forget to rate it up!