Check the fitting instructions for the drill size it's important to get this right as there isn't much room for error.The viewer comes in two pieces which locate into each other from each side of the door. They usually will accommodate various door thicknesses as the two parts screw together on a long thread.

Firstly, use a small drill bit (3mm.) to drill right through the door. Take care, It's important to get this straight and level. This will help guide the larger drill bit for the actual hole.
To drill the actual hole use a sharp flat drill bit and for best results follow the tip below.