Welcome to the Steam achievements guide for Psychonauts, in this guide I will provide pointers and explanations for just the achievements.
These are past guides that I have used for reference while building this guide.

• JagYui's achievements guide on Gamefaqs[www.gamefaqs.com]
  
• PinkCloud's guide on the old Steampowered forums Dead Link, outdated and some incorrect info
  
• My 2.0 guide on the old Steampowered forums Dead link, the old forums have been discontinued.
  
• Also, Fandom has a version of my guide as well[psychonauts.fandom.com]

If you are just getting started, here are a few things to note:

• It should go without saying, but this guide will contain spoilers, so proceed with caution!
  
• There is a point in the game known as The Point of No Return (PoNR) where you cannot return to collect certain items, these will be marked with this symbol: ⚠
  
• Some achievements can be missed if you progress too far in the story, these will be marked with this symbol: 🛑

As mentioned above there are some achievements that can be missed, here are the points a save should be made to minimize backtracking:

• Immediately after completing Sasha's Shooting Gallery
  
• Immediately after completing Milla's Dance Party
  
• Right before entering Edgar's Mind (Black Velvetopia) or more specifically, before turning all the cards into Edgar.
  
• Right when Raz reaches the top of the Asylum. or more specifically, right when you get Mr. Pokeylope.

Any corrections or suggestions for improvements are welcome.

Now lets get started!

These are guaranteed as you progress in the game, for help on any of these levels, see any of the other Steam walkthrough guides or check YouTube.

Complete Basic Braining

Complete Sasha’s Shooting Gallery

Complete Milla’s Dance Party

Complete Lungfishopolis

Complete The Milkman Conspiracy

Complete Gloria’s Theater

Complete Waterloo World

Complete Black Velvetopia

Complete The Asylum

Complete Meat Circus

These come as you level.

Achieve Rank 20

Achieve Rank 40

Achieve Rank 60

Achieve Rank 80

Achieve Rank 101

Pre-PoNR, you should have 45/50 bags, 6 PSI cards and be Rank 98 (assuming you completed the punching mini-game, 97 if not.) Meat Circus provides the 3 remaining levels. (be sure to visit the fortune teller after getting the three cobwebs.)

Any post-PoNR collectable items should be collected before exiting the Tunnel of Love in Meat Circus. If you aren't careful, you could accidentally throw yourself into the final boss battles.

This should be the hardest to achieve, this one requires 100% completion of the following:And that's why it's called Math is Hard

Edit: Holy cow, 12 years later it was pointed out that there are 5300 points of Figments, not 1600; and memory vaults and brains do not count. Shout out to Dr. Rank on PsychoPedia for pointing this out.

Simply put, collect all the related items.

Props to ZomB Turtles for having a very complete guide for collectables:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1712037819
Earn all Merit Badges

Most will come as you progress in the story, and a select few you need to pick up from Agent Cruller when you hit Levels 10, 20, and 30. The game cannot be completed without getting all the badges.

Find all Golden Helmets

All but one can be retrieved at any time in the following areas:

• Brain Tumbler Experiment - (after Sasha's Shooting Gallery) behind the thorny tower.
  
• Lungfishopolis - on top of a skyscraper after the monorail tunnel.
  
• The Milkman Conspiracy - You need telephone access, it's on the island accessible from the telephone pole after the road crew.
  
• Waterloo World - On the pink bridge near the peasant who is unarmed.
  
• Black Velvetopia #1 - Hang the window painting on the upper ledge at end of the of an alley near the Queen of Diamond entrance, enter the newly created window.
  
• 🛑Black Velvetopia #2 - THIS ONE CAN BE MISSED if you complete Black Velvetopia. Get the confusion grenade merit badge and use it to temporarily stop the bull so the dog can paint the poster on the wall, approach the painting* then return to Dingo Inflagrante for your reward.
  *There have been reported incidences where if you don't approach the painting after it has been painted, Dingo won't think it has been painted.
  
  BTW: you don't need the PSI Blast or Confusion Grenade upgrades for any achievements

The next five achievements are too complex to put all into this guide, I recommend visiting GameFAQS[www.gamefaqs.com] or Steam Guides if you want to track down any of those hard to find items. I will link any Steam guides that show up.

Redeem all 16 Scavenger Hunt Items
All found around the camp area and the Asylum, return them to Ranger Cruller

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=817867807

Re-brain the Children

All found at the Asylum, return them to Agent Cruller in the the sanctuary. Note: By this time, Agent Cruller will have moved down the bright hallway to the observation room.
---

These next three are found in the various minds, you can check your status in the collective unconscious area:
Crack all Vaults

Find and crack all vaults in each mind (most all minds have two, Meat Circus has only one)

Sort all Emotional Baggage

Every mind has five emotional baggages, find the corresponding tag and make 'em happy.

Gather all Figments
This is perhaps the second most painstakingly annoying achievement in the entire game.

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=407472200
Cook and consume two different kinds of roast in one sitting.

Use pyrokinesis, (available once you reach level 10,) to roast two small creatures in the same area, squirrel being one and a regular bird as the other(not a seagull or crow).

🛑Be sure to do this before the third brain tumbler, daytime is required as there are no birds at night.

Best area IMO to do this is on the pathway between the lodge and the parking lot.
This can be tricky as you have to sneak up on the creatures.

(hidden achievement)
Buy an item from the Camp Store on Christmas.

Date dependent, you need to buy something from the camp store on Christmas Day. Be sure to play the game on December 25.

Or, you can set your PC system clock to Dec 25 and get the same effect. BTW, Despite setting your PC system clock, the achievement will still show up as unlocked on the current day and not Dec 25.

For lack of a better term, these depend on Raz being there at the right place and time to witness or do something, some can be missed if you progress the story too far.

Witness Maloof's transformation

Immediately after Sasha's Shooting gallery; don't do the 2nd brain tumbler just yet, instead go to the parking lot to find Maloof, talk to Maloof.


Uncover Milla's Secret

When you start going up the first vertical climb area in Milla's Dance Party, you will see a inconspicuous looking balcony, float on down to it to find something very unsettling.
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=200906521

Hear Vernon's Ghost Story

Immediately after completing Milla's Dance Party, head to the lodge TV room to find Vernon telling a rather mindless tale. Hang around and listen to the entire tale.

Spy on Bobby's love life

Also immediately after Milla's Dance party, Go to the GPC Wilderness area and find Nils and Chloe, talk to them.

Find Edgar's Secret Garden

While in Velvetopia, purchase the Vine painting in the diamond alley, enter the sewer and find the gate that is opened via a telekinesis activated button. Once you climb out of the sewer, hang the vine painting at the end of the alleyway, climb the vine and follow along until you get to a secret garden.

Introduce all Camp Kids to Mr. Pokeylope

This assumes you re-brained all the children. When you get Mr. Pokeylope, DON'T take him to Shegore just yet. Instead, go back to the camp, find all the children and show Mr. Pokeylope to them.

Locations:
• Kid's Cabins JT, Chops, Dogen(around the grounds,) Kitty, Franke(girls cabin,) Bobby and Chloe(Coach's landing)
  
• Main Lodge: Nils and Elka (back lodge patio area)
  
• Main Lodge(Indoors): Phoebe, Quentin (playing) and Vernon(at a table).
  
• Parking Area: Mikhail and Maloof (Coach's SUV)
  
• Reception area: Clam and Crystal (Janitor Cruller)
  
• Lake: Elton and Milka (On the dock)
  
• Benny is nowhere to be found, but doesn't count against the achievement

Revisit all brains after completion

After meeting each mind's main objectives, be sure to re-enter the mind from the Collective Unconscious once more.

This can be easily be done after winning the game; A final autosave is made that puts you back in meat circus, use the teleport critter to teleport to the collective unconscious, if you are 100% you will also see a special message too!

Alternatively, load up the Point of No Return save, begin the ending battle sequence, and then use the smelling salts. Using the smelling salts post-PonR will deliver Raz to the Collective Unconscious rather than the camp, which also comes in handy for the levels that don't have Teleport Critters

These involve repeating a certain task over and over.

Read many bulletin board messages

Self explanatory, find a camp bulletin board and activate it until the achievement activates

Use the bacon. A lot.

Also mostly self-explanatory. The trigger takes place when you repeatedly ask Agent Cruller things after activating the bacon.

(Previously I said activating the bacon from Raz's inventory was the trigger, evidently this is not the case, per the comments)

Discover the secret history of Whispering Rock

In the middle of the camp parking lot is a giant log where the rings will detail the history of Whispering Rock, activate the front of the log until the achievement activates.

Give the Coach's speech on the stump

Go to the reception area where the campfire is, stand on the stump and press your use button to recite Coach Oleanders's speech he gave at the beginning of the game.

See yourself through the eyes of many others

Use Clairvoyance on at least 13 unique non-hostile 'creatures/things'. (Per Zak in the comments)

Complete the Punchy Target mini-game

In the second stage of Basic Braining is a punching mini-game that must be completed once to progress forward. This mini-game needs to be done 5 times total for the achievement, you will know you are on the final stage when it requires you get 50 points in 75 seconds.

Raz's PSI punch has a good enough reach that you should stay on top of the button once targets only appear in the inner ring.

NOTE: this seems to be much easier done with a good controller rather than KB+mouse