This is how I built a custom holster for my s&w 686 revolver. The scraps were procured at hobby lobby for 8$. It's not tooling leather but does the trick. Be safe when handling fire arms, I claim no responsibility for your stupidity!
Step 1: Tools And Materials
Tools: 4 hole punch, multi- hole punch, hammer, carving knife, edger, leather/wood burner, rivet and snap setter, crush rivet gun, needles, torch, belt sander.
Materials: scrap leather, nylon thread, leather cement, crush rivet, rivets, snaps.
Step 2: Cutting And Sizing
Scrap leather from hobby lobby can be very different from package to package. You need to find something that will work for you best ( or multiple pieces). Fold the leather around the pistol to ensure a proper fit. Trace around where you want to cut. Be sure to leave plenty of room for stitching. Plan out any straps or belt loops as well. Wet the leather thoroughly. With a hammer punch the stitching holes. Encase the pistol in Saran Wrap and tinfoil to make it water proof. Using clamps, sinch the leather around the pistol snugly and let dry.
Step 3: Embellishing
I did a few custom thing to this holster. First I I carved and burned the model of the pistol into the strap. It was a little difficult because of the type of leather, but came out well. I also added a lighter color leather upper. Finally, my favorite, I used the end of a 38 special Casing as the front snap button. To fix the casing to the snap with a crush rivet. I had to make an adapter to make the rivet go in correctly.
Step 4: Stitching And Rivets
I prefer to stitch by criss crossing the thread, as pictured. One thing I have a problem with is tying off the finished thread, so with nylon thread, you can melt it together. Be sure to use leather cement in between your seams for added strength. Punch any holes and rivet the holster as needed.