Step 1: Go to your favorite hardware store, and buy a wide angle door viewer.
You should be able to find one of these at any hardware store. I got mine at Home Depot for about $11. The key attributes that you're looking for is the largest possible "eye hole". There are a bunch of these door viewers that have very small openings, and they won't produce a very good image. Get the one I'm showing here, and it will work great. The opening is just about the size of the lens on my old Canon S230.