Welcome to Achievement guide for 100% completionist. First of all I'm sorry if there is any mistake in text. English is not my main language.

I made this guide cuz all other guides were little to hard to understand. This one will be hopefully a little easier to get. I recommend saving game often, so if you do something wrong you can just reload it.

Wake Up Call
Survive the manual override
Story related, cannot be missed.

You Monster
Reunite with GLaDOS
Story related, cannot be missed.

Undiscouraged
Complete the first Thermal Discouragement Beam test
Story related, cannot be missed.

Bridge Over Troubling Water
Complete the first Hard Light Bridge test
Story related, cannot be missed.

SaBOTour
Make a break for it
Story related, cannot be missed.

Stalemate Associate
Press the button!
Story related, cannot be missed.

Tater Tote
Carry science forward
Story related, cannot be missed.

Vertically Unchallenged
Master the Repulsion Gel
Story related, cannot be missed.

Stranger Than Friction
Master the Propulsion Gel
Story related, cannot be missed.

White Out
Complete the first Conversion Gel test
Story related, cannot be missed.

Dual Pit Experiment
Do the same test twice
Story related, cannot be missed.

Tunnel of Funnel
Master the Excursion Funnel
Story related, cannot be missed.

The Part Where He Kills You
This is that part
Story related, cannot be missed.

Lunacy
That just happened
Story related, cannot be missed.

Drop Box
Place a cube on a button without touching the cube
Can be done in test chamber 6, when droping cube on jump-pads use portal right away. One on platform u already used and one above the button.

Final Transmission
Find the hidden signal in one of the Rat Man's dens
Hidden room can be found in test chamber 6

https://www.youtube.com/watch?v=bTXCcp0hPMA
Pturretdactyl
Use an Aerial Faith Plate to launch a turret
Can be done in test chamber 6 or 15, just place turret on jump-pad.

Preservation of Mass
Break the rules in Test Chamber 07
All u need to do is bring companion cube to the exit door of Test Chamber 7

Overclocker
Complete Test Chamber 10 in 70 seconds
Self explanatory

Good Listener
Take GLaDOS' escape advice
When GLaDOS asks u to finish last chamber while escaping, do it, instead of running.

No Hard Feelings
Save a turret from redemption
During chapter 5 you'll move on conveyor belts, on second belt there will be turret with functioning laser, take it with you.

Scanned Alone
Stand in a defective turret detector
In chapter 5, when asked to remove turret from scanner, let yourself be scanned.

Door Prize
Examine all the vitrified test chamber doors
Can be found in chapter 6.

https://www.youtube.com/watch?v=4ECCRu4hgbQ
Schrodinger's Catch
Catch a blue-painted box before it touches the ground
Can be found in chapter 6. You'll need to use blue gel on cube in box, save game before this and use the gel

Ship Overboard
Discover the missing experiment
You'll need to jump through giant O in chapter 6, but instead go to the highest room and on left wall in corner there will be hidden door. Walk through them to the end.

Portrait of a Lady
Find a hidden portrait
In chapter 7 after using speed-gel to reach button near the celing, right after blue gate, turn left and behind scaffolding u'll see white wall, place portal on it and place second one to your right, going through will lead you in upper room with portrait, GlaDOS will comment on it.

You Made Your Point
Refuse to solve the first test in Chapter 8
Don't do anything in Wheatley's first puzzle room.

Smash TV
Break 11 test chamber monitors
Break all Wheatley's monitors, starting chapter 8.

https://www.youtube.com/watch?v=IqDd_1PItjc
Pit Boss
Show that pit who's boss
In chapter 9 after escaping Wheatley's trap with spikes, go back and do as he says.

High Five
Celebrate your cooperative calibration success
Story related, cannot be missed.

Team Building
Complete all test chambers in the Team Building co-op course
Story related, cannot be missed.

Confidence Building
Complete all test chambers in the Mass and Velocity co-op course
Story related, cannot be missed.

Bridge Building
Complete all test chambers in the Hard-Light Surfaces co-op course
Story related, cannot be missed.

Obstacle Building
Complete all test chambers in the Excursion Funnels co-op course
Story related, cannot be missed.

You Saved Science
Complete all test chambers in all courses of co-op
Story related, cannot be missed.

Rock Portal Scissors
Win 3 co-op games of rock-paper-scissors in a row
Self explanatory

Air Show
Perform 2 aerial gestures before touching the ground in co-op
Can be performed during long jump or while infinite falling.

Asking for Trouble
Taunt GLaDOS in front of a camera in each of the five co-op courses
Can be completed any time, just use gesture in front of camera in every course.

Narbacular Drop
Place a portal under your co-op partner while they are gesturing
Self explanatory

Empty Gesture
Drop your co-op partner in goo while they are gesturing by removing the bridge under them
Can be done in level with hard light bridges.

Can't Touch This
Dance in front of a turret blocked by a hard light bridge in co-op
Self explanatory

Gesticul-8
Perform all 8 gestures of your own volition in co-op
Can be done after obtaining all gestures

Friends List With Benefits
While playing co-op, hug 3 different people on your friends list
I hope u got friends....

Four Ring Circus
Enter 4 different portals without touching the ground in co-op
Can be done in course 1.

https://www.youtube.com/watch?v=6DXNccrNeag
Party of Three
Find the hidden companion cube in co-op test chamber
Can be found in final area of excursion funnel section

https://www.youtube.com/watch?v=Yz8Z0PWxSUQ
Triple Crown
Solve 3 co-op chambers in the Mass and Velocity course in under 60 seconds each
The easiest stages are 1,2 and 4, so plan your route and SPEEDRUN !

Portal Conservation Society
Complete Chamber 3 in the Hard-Light Surfaces co-op course using only 5 total portal placements

https://www.youtube.com/watch?v=YbJTkuTSZA4
Iron Grip
Never lose a cube in Chamber 6 of the Mass and Velocity co-op course
This one is difficult, so once again make a plan. Host can reload map using command changelevel mp_coop_multifling_1

Talent Show
Never lose a cube in Chamber 6 of the Mobility Gels co-op course
Once again plan your route, if you fail let the host use
changelevel mp_coop_paint_red_racer to reload map.

Still Alive
Complete Course 4 with neither you nor your co-op partner dying
Plan EVERYTHING !! Also here is command for reloading map
changelevel mp_coop_tbeam_redirect

Professor Portal
After completing co-op, complete Calibration Course online with a friend who hasn’t played before
Self explanatory

Good luck in collecting achievements. I hope this guide helped you a little bit, if yes then you can give it a thumbs up so everybody can see how to collect everything.

I took inspiration from The Horror Network.
All videos added in this guide are made by someone else.
Videos are from IGN, GX, GamesRadar, ThatGuyTaylor.