A basic overview of snaps appears
here.
There are two basic types of snaps: post style and prong style. The best type for use on nylon holsters is the post type which requires making a single hole in the holster cover and another mating hole in the holster body.
Each snap is composed of 4 pieces (see attached photo):
There are a few ways to make the holes in your nylon holster to accommodate the snap post. I used an electric drill, but you might melt a hole with a heated awl or nail instead. Be sure the hole isn't larger than the snap's post diameter or else the snap will fit too loosely.
I placed a small block of wood inside the holster before drilling to avoid drilling through the entire holster and out the back.

Some inexpensive snap assortment kits come with a pin punch-type installation tool which you hit with a hammer. I found that difficult to use and easy to smash your fingers. Instead, I use a pliers style tool similar to this
Dritz® Mighty Snap-A-Plier For Heavy Duty Snaps:
http://www.joann.com/joann/catalog/productdetail.jsp?CATID=cat2851&PRODID=prd34657I have 2 sizes of snap, each with different length posts. If the nylon material is too thick, the shorter post snaps won't reach through. If you have snaps with a post which is too long, you can either shorten the post using a Dremel tool and cutoff wheel, or just install a washer as a shim. The latter workaround is easier, though using a shim washer can cause the snap to protrude a little further from the material.
Suggest you practice first on a scrap piece of nylon or leather.