Choose a position for the peephole, considering the average height of a person (5ft 7”) and the height of those who will be opening the door and using the peephole. Mark this position with a pencil and drill a mounting hole there using a drill bit that is a fraction larger than the peephole. To avoid splintering the door, drill very slowly and stop just as the drill bit breaks through the surface at the far side of the door, then drill through from the other side.