Measure the distance between the mounting holes on either side of the projector screen. Slide the stud finder across the wall onto which you plan to mount the screen and mark where the finder beep---these are studs that are safe to drill into. Select two studs that are the same distance apart as the mounting holes on the screen and mark them each with an "X."
Hold a 2-foot-by-4-foot board between the marked studs at the height you would like to mount the screen. Place a liquid level on top and adjust the board until the bubble in the level is centered. Mark the top of the board at each of the studs.
Drill into the markings on the studs. Line the projector screen's mounting holes up with the holes and screw it into the wall.