To get to the concealed room in the attic, the player has to:

1) activate the doorway portals by reading the note upon entering the house;
2) then step through the doorway portals, starting from the ground floor and gradually moving all the way up to the attic.

However, before stepping through, the portals must display the correct scenes on the other side, otherwise the portals will deactivate, forcing the player to return to the start. Entering the correct scene will trigger a white flash, while entering a wrong one will trigger red.
This video guide eliminates the process of trial and error involved in finding the correct scenes.

Duration: 3:40

https://youtu.be/1d6hn7W9akU