The house the investigators break into, the manor where the guests were murdered, the asylum with the locked wing: draw it room by room, with walls that follow the grid, doors and windows that snap onto them, and the furniture where the clues are.
A trapdoor under the rug or a room the architect left off belongs on a layer for the game master, and the copy for the players leaves it out. Print it as a 1920s architect’s drawing or a pencil sketch, A4 or A3 at 300 DPI.