Of the nine achievements that come packed with The Door in the Basement, three are tied to finding all the collectibles (36 notes and 8 discarded eyes) throughout the game. This guide focuses on the said collectibles and lists them in order of gameplay progress. The remaining six achievements, which unlock naturally through progression, are utilized to divide the guide into sections to help the player keep a better track of the items in question.
Similarly, the points where one comes across the character Sibyl (five in total) are also mentioned since the player needs to give her the discarded eyes he/she finds throughout the game: not only to progress towards the related achievement but also to learn more about the world.
The descriptions for the notes are as provided by the developers themselves, in this post.

Note #1 - On a table before dropping down into the sewers.
Note #2 - On a table after the timed gate in the sewers.

Note #3 - In the blue-lit room after dropping into the caves.
Note #4 - At the back of the large room immediately after the previous note.
Note #5 - On a crate just before the large door that takes three keys.
Note #6 - On a crate outside the next safe room.
Note #7 - On a steel drum in a corridor after crawling through a small tunnel.

Note #8 - On a table beside the TV after re-entering the sewers.
Sibyl Encounter 1/5 - As you step into the tunnel, you come across a talking hand. Her name is Sibyl. Every time you deliver her a discarded eye, she rewards you with a few lines of lore. You can return to her after getting the next collectible, the first eye, or you can talk to her in one of the following encounters and deliver all the eyes you collect up to that point.
Discarded Eye 1/8 - Under a table in the room right after the one with two large fans.
Note #9 - On a table just outside the fan control room.
Note #10 - On a table in the room just after the fans.
Note #11 - On a stack of crates in the cave leading up to the walkway controls.
Note #12 - On a stack of planks immediately after crossing the walkway.
Note #13 - On a crate full of doll parts after the tunnel collapse.

Note #14 - On a table in the corridor after dropping into the waterworks.
Note #15 - On a table in the corridor with the large fan and pipes.
Note #16 - In the room adjacent to where you find the flashlight.
Note #17 - On a table in the dark corridor through the door after picking up the flashlight.
Discarded Eye 2/8 - On the shelves in the second room after the dark corridor.
Sibyl Encounter 2/5 - After solving the water pipe puzzle you find Sibyl under the stairs.
Note #18 - On a table in the room with the TV.
Discarded Eye 3/8 - On a table in the first room on the left after unlocking the gate.
Note #19 - On a table in the corridor after unlocking the gate.
Note #20 - On a crate next to a wall to your left after entering the abattoir grounds.
Note #21 - On the table next to the valve handle.
Note #22 - On a crate just before picking up the ladder segment.
Note #23 - On a table inside the first wooden walkway.
Discarded Eye 4/8 - On the floor next to the safe room after climbing the long ladder.
Note #24 - On the shelves just before leaving the area.
Note #25 - On a table in the room with the elevator.
Sibyl Encounter 3/5 - Sibyl waits for you in the run-down corridor across the elevator.
Discarded Eye 5/8 - On the floor next to the locked door in the following corridor.
Note #26 - On a table just after the fusebox.
Note #27 - On a crate just after the keypad door.
Note #28 - On a table in the room with the three fans.
Discarded Eye 6/8 - On the shelves in the room adjacent to the one with lots of cages.
Note #29 - On a table in the room where you crawl under the floor.
Note #30 - On a table in a room after picking up the crank handle.

Note #31 - On a crate to your right after the elevator crash.
Assuming you collect all the notes up to this point, this achievement should unlock in the next safe room you go in, with a total of eight people shown on the board.

Note #32 - On a steel drum next to a cage full of people after you wake up in a cage.
Note #33 - On a crate to your right after crossing the makeshift bridge.
Note #34 - On the table next to the gate key.
Discarded Eye 7/8 - On a pallet in the corner in the same room with the gate key.
Sibyl Encounter 4/5 - You come across Sibyl once more after unlocking the gate.
Discarded Eye 8/8 - On the table under the stairs that leads to the overlooking room.
Note #35 - On a steel drum just after opening the gate with the valve and crouching under the boarded-up doorway.
Sibyl Encounter 5/5 - You meet Sibyl for the last time just after passing through the grinder.
This achievement should unlock once you deliver all eight discarded eyes to Sibyl.
Note #36 - On a table in front of the door after going up the elevator.
This achievement should unlock once you read the thirty-sixth one.
Feel free to comment if you require any further assistance.