Knock, Knock -Who’s there? Installing a door viewer (peep hole) in you door will keep you from asking that question as you will be able to see who is on the other side without asking or opening the door to find out, keeping you safe until you know who it is.
Installing a door viewer is fairly simple task, needing just a few basic tools, such as a drill, drill bit and screw driver.
A door viewer can be installed in a wood, metal or fiberglass door.
Check out this short for instructions on installing a peep hole.
I think anything about do-it-yourself is great