Slender: The Arrival

In the game, go to the Extras-->Stats section. Statistics are written here on difficulty levels and the time spent completing one level. Your total time must be less than 45 minutes on one of the difficulty levels. For example, on easy difficulty you completed the prologue in 13 minutes. Start this level on easy difficulty and complete it much faster. For example, in 5 minutes. At the end of the level, your 13 minutes will be converted into 5 minutes.

Thus, you need to replay the levels so that the total time is less than 45 minutes

There are 10 places on the level where notes can be found. And because there are 10 places, and 8 notes, then they cannot be in 2 random places. Advice: first, run through the entire map in search of exactly those places without notes in order to build your route in advance. Don't worry, you will have about 5 minutes (which is more than enough) before Slenderman appears. And remember, with each note he becomes more and more wild. Don't be afraid that he can teleport directly to you and send you to go through the level again.
PS If you leave the building with the labyrinth towards the iron water tanks and go a little further straight along the path, you can find a large stone, this is also one of the locations where the note may be located. Due to the color palette and size of the note, it can be easy to miss it there