Get a cap that's big enough to fit around the lense of your camera.

Drill a hole in the middle using a bit that's the same size as the peephole you're going to use. Don't go too big!

Clean up the edges with a chisel if you need to.

Measure around the inside of your cap and cut out some foam strips to place inside so the cap fits snug on your camera.

I didn't glue mine in place or anything, but you definitely could. It might be a good idea at this point to paint the inside of your cap black so it's nice and dark when you look through your camera. I didn't and there's a bit of a blue ring around the edge where the light kinda goes through the plastic.

There's a casing that the peephole screws into that you'll need to take off.<BR>Screw your peephole into the cap, but make sure not to go too far in that you touch the lens on your camera.

Stick it on your camera, adjust, and that's it.<BR><BR>Here's my hallway...

This is the same camera position, but with the fisheye.