Like Clay has 8 achievements:

• 5 story progression
  
• 1 miscellaneous
  
• 1 bad ending
  
• 1 good ending

The miscellaneous(?) and bad ending achievements will require a new play through if the good ending is achieved before those two. These are mentioned in the guide below as well in case you forget.

There are plenty of checkpoints so dying never sends you back far. Which is good since there are times when you are randomly killed by enemies.
The game only saves your progress on loading a new level. This means if you quit partway through a section, you'll have to redo it from the start next time.

Objective: "Find Tom Kemer"

• Check your mail: J. Stevens.
  
• Head up to your apartment, it's on the 5th floor.
  
• Check two notes. One tells you to go to Daniel Berger's place on the 2nd floor.
  
• Take the piece of wire next to one of the notes.
  
• Go to Daniel Berger's apartment on the 2nd floor. Check the note - tells you to check Daniel's mailbox.
  
• Back to ground floor, peek into D. Berger's mailbox. Use piece of wire to get key.
  
• Up again to Berger's apartment. Head inside.
  
• Check messages, the note on the table. There are two more in the sleeping quarters.
  
• Do as note says and play the gong (in the corner) five times.
  
• Check the box on the table. Flip it over. Press the buttons in "all corners". Open the flap and take the key.
  
• Unlock the last door in the apartment, pick up the first ring and give it a kiss.
  
  Level 1 Member
• Follow the wall message's advice and go to the chapel in the basement.
  
• Lots of storerooms down here (but nothing of interest). Find the one that leads to the chapel.

Objective: "Find Tom Kemer"

• A message tells you to "wash yourself" first. Follow the arrow to the shower room and use the showers. See message: "1 dice 3 pond 5 roof".
  
• Return to the hallway.
  
• Find the door to "Learn the word of Sor"
  
• There's a note to read in the wall cabinet.
  
• Use the book in the left cubicle to pray. See message: "2 wall 4 roof".
  
• Combine the two messages and use it to unlock the door.
  "1 dice 2 wall 3 pond 4 roof 5 roof"
  Dice = square; wall = pipe character; pond = circle; roof = caret
  
  
• Walk down, find the secret door switch behind the lectern.
  
• Listen to the recording if you'd like otherwise head inside.
  
• Inside, read the notes on the desk, listen to the answering machine. Eventually one mentions a door code: 1887.
  
• Further inside, use the door code to unlock the door.
  
• Proceed downstairs, find the second ring and kiss.
  
  Level 2 Member
• Exit the way you came, back to the main apartment building.

Objective: "Enter the Hall of Trials"

• Find the door to the Hall of Trials. It's one of the doors on the basement floor.

Objective: "Find Tom Kemer"

• Take the elevator down.
  
• Down the hall, through the door at the end to "Registration".
  
• Listen to the answering machine. See the note on the wall. Four keys are missing. Guess we know what we have to do next.
  
• Enter the doorway. See two more doorways: "Walk of Sin" and one unmarked.
  
• Follow the "walk of sin". Tells you to run around "min 5 per day". Do so, avoiding the spider attack on the second rotation. Repeat until the spooky music stops.
  
• Follow the unmarked hallway now.
  
• It'll lead to an area with three desks. Open the book on the middle one for a key.
  
• Go down the stairs.
  
• Find the soap on the green shelf. Rotate to read the code underneath: 4428.

Objective: "Study Sor's holy relic"

• First of all, open the first aid cabinet to find a key.
  
• Open the wall safe with 4428. Another key and note.
  
• Listen to the recording. It tells you to "stare at the relic long enough, you will be enlightened."
  
• After doing so, enter the "chamber of fire".
  
• Exit, go back upstairs.

Objective: "You may now leave the Hall of Trials"

• Go back to registration.
  
• Along the way, two pairs of annoying spider legs will attack you. There isn't much strategy to getting past them, it's seems random and they have massive hitboxes. The checkpoint is only back at the stairs, so keep at it. Use the walk of sin hallway to bypass the second pair.
  
• Open three lockers with your three keys. All three contain notes. One note in particular tells you the safe code 5219.
  
• Use 5219 to open the safe next to the prison door.
  
• Take the key and unlock the prison door.
  
• Enter the first cell and take a key from the cabinet.
  
• With the final locker key, find it for another note.
  
• There's nothing else here for now. Return to the elevator.

Objective: "Re-enable power to the elevator"

• Go to the prison cells, next to them is a breaker. Use it.

Objective: "Punish yourself in the name of Sor"

• When the exciting music starts, find the nail on the bench outside the cells.
  
• Use it to stab yourself until everything calms down.
  
• Return to the elevator.

Objective: "Investigate the High Priest's apartment"

• At the stairwell will be two handy messages:
  
  • "The High Priest's apartment is located on the fourth floor"
    
  • "I saw one of the children playing with one of the keys"
• We'll find the key first. It's on the ground floor, there's a larger open area with some building blocks. Among them is the key.
  
• Up to the fourth floor, the high priest lives in the last door on the left.
  
• Three doors, one leads to the bathroom, one locked with a weird eye, and one goes to the main room.
  
• In one of the document trays on the desk is a note. It mentions a safe code: 8614.
  
• Use 8614 to open the safe. Two more notes - just backstory.
  
• Enter the side room. Read the instructions. You must: "take the weight representing the person or servant who is dearest to Sor and put it on the scale." Then "step on the wooden platform."
  
• The correct answer is weight with the cross (or I think it's Sor's brother).
  
• Go back to the weird eye. Head inside.
  
• Listen to the recording if you'd like. Otherwise find the ring and kiss.
  
  Level 3 Member
• Exit the apartment.

Objective: "Enter the Library in the basement"

• Self-explanatory by now, find the new door in the basement.

Objective: "Find Tom Kemer"

• Follow the linear path to "Registration".
  
• On the desk is another answering machine and note in the tray with more backstory.
  
• Circle the area once to get your bearings. There are thee doors: Labyrinth, one near the Kroner Heating System, and Test of Faith, as well as a large staircase in the centre.
  
• Enter the Test of Faith.
  
• Open the cabinet for a note. It mentions the safe code 8996.
  
• Only one door here is unlocked.
  
• Enter it and press the button. Bright lights warning? I think we can just leave after pressing the button.
  
• Go down the large staircase now.
  
• On the first floor down is the 8996 safe with a note inside.
  
• One of the desks on the floor second from the bottom has a note hidden inside a book.
  
• On the lowest floor is another recording.
  
• I'm not sure if the confessional does anything.
  
• Either way, enter the waiting room.
  
• Find a note inside. It tells you to take the screwdriver and check the Kroner Heating Systems' box for messages.
  
• Well, take the screwdriver then. There's also another answering machine for more backstory.
  
• Open the Kroner Heating Systems' box.
  
• Inside is note that mentions a code: 705--O. And we'll need to fill in the blanks.
  
• Return to the waiting room, and use the code 70562O. This opens a hidden room.
  
• Find a key and a note.

Objective: "Tom is not here, advance to level 4 in the labyrinth"

• You should have found the labyrinth door earlier, so go there now and unlock the door.

Objective: "Survive the labyrinth"

• Follow the path. Press the button and listen to its instructions. I'll paraphrase:
  
  1. Break twig
     
  2. Run to the end of the labyrinth
     
  3. Kiss ring
     
  4. Return to library
• Proceed with step 1.
  
• When the path opens, you'll see the labyrinth.
  
• Note the exit signs pointing left. That's where you'll exit when you come back since the way you entered is blocked.
  
• There are enemies in here that will kill you, but the correct path is laid out for you with arrows.
  
• At the end of the labyrinth is an arrow shooting trap. Stepping on the tiles at the intersections will release arrows. Just hide in the alcoves to avoid the arrows as you proceed forward. Note one of the alcoves hides room with the next ring.
  
• Run to the last alcove to find a switch that unlocks the room with the ring.
  
• Run back to the ring while avoiding arrows and kiss the ring.
  
  Level 4 Member
• Avoid the trap and run back to the labyrinth.
  
• Backtrack through the labyrinth and to the exit I mentioned earlier.

Objective: "Find out what happened to Tom Kemer"

• You'll exit in the Test of Faith.
  
• The door next to the Kroner Heating Systems' box will now be open.
  
• Follow the linear path inside.

Objective: "Past the test on the upper floor and pick up the key to the Inner Sanctum"

• In front of you is an elevator. Take it to floor 1.
  
• (The other path here leads back to where you entered the library and where you will be exiting later.)
  
• In this dark chamber is another puzzle.
  
• The puzzle doesn't make much sense, but you can only move when there's light and you need to stay still when it's dark.
  
• Follow the path to the end and ring the instrument.
  
• Go back the way we came, grabbing the key from the box with the arrow on the way out.
  
• Take the elevator to floor 5.
  
• There are two notes here as well.
  
• Take the elevator to the upper ground floor.

Objective: "Go to your apartment, grab your stuff and leave the building"

• There's a note in the wall box on the way out.
  
• Take the shortcut and back to the apartment building.

Objective: "Go to your apartment, grab your stuff and leave the building"

• Return upstairs, following the linear path.
  
• After you reach the dead end, go back downstairs, following the red candles.
  
• Enter the door between two candles.
  
• Take the elevator up.
  
• Down the corridor, open the door marked with candles and into the red door.

Objective: "Find a way out"

• Another "registration" area. There's a note and answering machine on the desk. Note the 4125 scratched into the table.
  
• Use the code to open the safe. Inside are two more notes.
  
• Go downstairs.
  
• There's not much in the upper area except another recording.
  
• Head down more stairs.
  
• "Wait for the green light…", yeah ignore that and take the elevator down.
  
• Follow the linear path to a puzzle.
  
• Stand on the switch that looks like a spider (i.e. has eight legs).
  
• Down another corridor. Straight at the intersection. Through the only unlocked door.
  
• The drawers contain nothing interesting. Go through the next door and watch the man jump.
  
• Follow him down.
  
  Took a leap of Faith
• Return to the walkway, take the key at the edge.
  
• Return to the intersection and down the other path.
  
• Unlock the door.
  
• Inside is a note and another ring.
  
  Level 5 Member
• Return to the room with the drawers and enter the catacombs.

Objective: "Explore the catacombs"

• Walk down, read the notes. One note is especially important as it provides hints. Specifically, it mentions:
  
  • a rock between closet doors
    
  • piece of wood which holds open the trap door
    
  • piece of wood next to the statue
• Additionally, the note says:
  
  
• Pull the blue book to open the closet.
  
• This is where the paths split for the endings.

You should go for the bad ending first to save a full play through. The game saves during the good ending, so you cannot reload back to before ending choice. (source)

• Enter the closet door, ignoring the rock by the closet.
  
• After passing a creepy statue, turn right and up the stairs. Follow that path to a lever.
  
• Look on the floor to see two candles. The line on the note earlier represents the lever, so this one should go straight down.
  
• Return downstairs and go the other way.
  
• You'll see a small area with a lever and a single candle. So this lever must go horizontally.
  
• Keep following the path down to the final lever accompanied by one candle.
  
• Return back the way we came until a statue with red eyes looks at you.
  
• Enter the doorway opposite the creepy statue.
  
• Before touching anything, explore this area and find four dials marked with: a grass, heart, rose, and sun symbol.
  
• The corridor opposite the Sun dial also has two notes. One says:
  
  • Grass 1x ←
    Sun 2x ←
    Rose 2x →
    Heart 1x →
• Follow the note and input the correct sequence. Memorise the path because as soon as you start, a statue will chase you so you need to be quick.
  
• A new path opens up in the hallway between the rose and sun dials.
  
• Press the hidden switch in the top shelf of the bookshelf.
  
• Follow the cave to a statue head.
  
• Read the note. Rearrange the sticks on the wall to form an upside-down cross.
  
• Jump down.
  
• The game will inform you the statue closed, locking in the bad ending.
  
• Follow the cave. It's quite dark here and there are a lot of pitfalls, but the path is linear.

Objective: "Defeat the Demon"

• Boss battle!
  
  • The boss will shoot red spiders at you. This is its only attack and you'll be avoiding these the entire battle.
    
  • Four slots will open up in the walls revealing four tablets. Use each one five times until it breaks.
    
  • Once one breaks, press a button that lights up on the rotating pedestal it's standing on. This will reopen the slots.
    
  • Repeat this pattern until all four tablets are broken.

Objective: "Go back upstairs"

• Go back through the cave you traversed to reach here… and you're stuck.
  
• Enjoy the monologue and the ending.
  
  Bad Ending

• Follow everything done in the bad ending, except with three additional steps:
  
  • At the wardrobe at the beginning, move the rock to hold the door open.
    
  • After solving the lever puzzle, take the stick opposite the creepy statue and use it on the door slot to prop up the door.
    
  • After solving the cross puzzle, pick up the block of wood nearby and use it between the statue heads before jumping down.
• After beating the boss, since all your exits are still open, retrace your steps all the way back to where you entered the catacombs.

Objective: "Go back to the main building"

• Things have changed a bit now, so feel free to explore a little. You're safe for the rest of the game.

Objective: "Go to your apartment and call the police"

• In case you forgot, your apartment is on the 5th floor.
  
• Using the phone in your apartment will trigger the final monologue and ending.
  
  Good Ending