Look at tk409.com. He has a great tutorial on snaps. The one on his site has photos.
Snap Tutorial:
I just finished adding snaps to my vest and it worked great! I traced around my armor on paper and cut it out (marking which side of the paper armor faces out).
Then, I cut some rectangular tabs of plastic (sintra), drilled a hole in one end and stuck the male snap through. For me, the rectangle tab is important, because I superglue the end without the snap (the tab) to the armor.
Using a blade, I scored (made several marks) both on the armor and the plastic piece in a crosshatch pattern. Then added superglue to the tab, and pressed it onto the armor.
Then I pressed the paper onto the armor (now with snaps) to make a mark on the paper (to know where the snaps are located.)
I laid the paper out on my vest exactly where I wanted the armor. Then I used a pencil to poke through the paper where the snaps should be located (twist it a couple times to mark the vest).
At the mark, use a big nail to poke a hole through the vest. Then assemble the female snap through the vest at the hole.