You will find notes throughout the game. These notes are randomized, and you will get different notes on each play through. You will not be able to attain all notes in one single play of the game, so this achievement may take some time. There are a total of 52 notes to collect. The best way to find them is by exploring every room on every floor.
Note: Once one of these notes has been obtained, you will not see it in the open again. Therefore, every note that you find in the game will be new, and count toward this achievement.