While writing this guide I tried to keep it as spoiler-free as possible lore wise.
Where the instructions would reveal some fundamental lore they are hidden with [ spoiler ] tags.
There will still be light spoilers so be warned.

Pretty self explanatory.

• Interact with the notebook as prompted at the start of the game;
  
  
• Use the stickers to decorate it as your heart desires;
  
  
• Confirm the changes.
  

When you first arrive at the town there will be three Lost Pies scattered around the area.

• Pick up at least one Lost Pie;
  
  
• Go talk to Victor, who tells you he is hungry;
  
  
• Try to give him a Lost Pie.

These achievements are connected: you have to complete BIG SPENDER to obtain EXPERT INVESTIGATOR.
In particular the EXPERT INVESTIGATOR achievement requires you to complete the notebook with all the profiles of the characters in the game:

• Lobster Cop will already be registered
  
  
• And the Detective aswell
  

So let's start (make sure you go through all the questions for each character):

• Talk to Susan to add her profile to the notebook;
  
  
• Talk to Ralph to add his profile to the notebook;
  
  
• Talk to Noddy to add her profile to the notebook;
  
  
• Talk to Victor to add his profile to the notebook;
  
  
• Talk to Mary to add her profile to the notebook;
  
  
• Talk to Carlos to add his profile to the notebook;
  

Obtaining the Money

• Pick up at least three Lost Pies (try giving one of them to Victor to obtain the GROSS achievement);
  
  
• Give the first three Lost Pies to Mandy;
  
  
• The remaining two will now spawn (they can spawn in places where there were already Lost Pies before);
  
  
• Pick up the last two Lost Pies and give them to Mandy;
  
• Mandy will give you 1 Money and a Fresh Pie;
  
• Give the Fresh Pie to Victor, who will give you a Party Hat in exchange and 1 Money for having talked to him;
  
  
• Go to Carlos and give him the Party Hat in exchange for one Rug;
  
  
• Give the Rug to Noddy who will give you 1 Money and Builder Tools;
  
  
• Give the Builder Tools to Ralph for 1 Money and one Hook;
  
  
• Give the Hook to Susan in exchange for 1 Money and the possibility to enter her house.
  

Now that you have the Money follow these steps before talking to Mary again.

• When you have at least three Money talk to Carlos;
  
  
• Ask to buy one of the photos in exchange for three Money;
  
• Grace Bruxner's avatar will interrupt you and refunds you the Money you so irresponsibly spent;
  
• A photo of Grace Bruxner will be added to the notebook (this is necessary to obtain the EXPERT INVESTIGATOR achievement).
  

• Talk to Mary and give her 5 Money in exchange for a Mysterious Phone Number;
  
  
• Go inside Susan's house and use the phone to call the Mysterious Phone Number;
  
  
• Barney will pick up the phone;
  
  
• After the call ends Barney's profile will be added to the notebook;
  
  
• Go out of the house and pick up the Glasses on the ground near one of the lampposts on the Warlock Woods entrance path.
  

Now have a last check to make sure you have all the entries on your notebook except for the Invisible Wizard:

Again, pretty self explanatory.

• After picking up the Glasses, enter the house between Carlos' shop and Victor's house;
  
  
• Look for the Invisible Wizard hiding between the cardboard boxes;
  
  
• She will teleport three times;
  
  
• After the third teleportation the second door in the back will open;
  
  
• Go through that door, go downstairs, to find the Wizard in a corner;
  
  
• Talk to her and her profile will be added to the notebook completing it.
  

After talking to the Invisible Wizard you have to decide what to say to the other citizens.

• Lie and accuse Evil Guy to obtain this achievement.
  

After talking to the Invisible Wizard you have to decide what to say to the other citizens.

• Tell the truth to obtain this achievement.
  

The FIBBER and SNITCH achievements cannot be obtained in one single run.
If you don't want to play again all the game to obtain the one you missed follow these steps:

• Go to the main menu
  
• Use the following shortcut:
  Left Shift + L + 4

WARNING: This shortcut will overwrite your save file deleting it. Use it only after beating the game once.
The game will load automatically at the start of the scene where you have to make the final choice, allowing you to choose the other ending.

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(Thanks to Crossbourne for suggesting the following shortcut in the comments)

Originally posted by Crossbourne:

If you've missed the Big Spender and Expert Investigator achievements, you can also use

Left Shift + L + 2

to drop at the entrance of town immediately. Skips the whole Lobster Cop scenes.

Same warning as the precedent shortcut applies.