To install a door peephole, measure the position you want to install the peephole, mark the area, drill a hole through the door, apply silicone to the inside of the hole, and install the peephole. A gaffer tape and drill are handy in the installation.
Purchase a peephole from a local home center or hardware store. Measure the height of the shortest adult in your household with a measuring tape, and subtract 1 inch from the measurement. Use the measurement to find the position you wish to install the peephole, and mark the position with a gaffer tape. From the inside of the door, drill through the door, pressing lightly and running the electric drill at full speed.
After drilling the layer on the outside of the door, strip off the tape. Unfasten the peephole to reveal two parts. From the outside of the door, slip the piece with the lens into the hole. Insert the other part through the inside hole. The inside portion of the peephole normally comes with a slot. Place a thin blade on the slot, and tighten the two parts together.
Alternatively, lightly apply silicone through the inside hole, and place the two parts via each side of the door. Wipe away any silicone on the surface of the door with a rag. Allow the silicone to dry properly before testing the installation.