The rivets I provide with buckles and mounts have been annealed and are very soft. Only a few tools are needed to add your buckle or mount to leather. The basics are:
Awl
Some sort of peining hammer: ball, straight, or cross.
Snips.
Anvil-any block of hard metal will do.
Secret Tool
The Secret Tool is a nail set, which can be purchased at any hardware store. The middle of a set of three is usually the best.
Poke a hole in your leather with your awl. For mounts, put the rivet though the leather from the back. This will give you a metal backing to rivet against. If you are attaching a buckle plate, you can rivet from either side. Place the leather and rivet on the anvil, and slide the mount onto the rivet.
Clip the rivet so that one to one and a half rivet diameters protrude. Wear protective eyewear or close your eyes and look away- the clipped off bit of rivet will fly in a random direction.
Hold the mount against the leather using open ships or pliers, and pein.
Once you have delivered a few hammer blows, the mount should be locked to the leather. The pliers can be withdrawn and the rivet hammered down causing the head to spread.
Don't hammer too hard, or for too long (practice will let you know when to stop). Unless you have hammered quite a few rivets, the rivet head won't be perfect. It will be rough and a bit irregular. Now for the Secret Tool:
One or two strikes will round and smooth the rivet head.
If you are making a complete belt, start with the buckle and strapend. I don't recommend using any measuring tools more complicated than a finger. Mark two spots, each a thumb-joint distance (or whatever your preference is) from the strapend or buckleplate edge. Add mounts to these spots.
Fold the belt in half, mark the half-way spot, and add a mount. Repeat until the belt is finished. Medieval spacing is somewhere between touching and every four or five inches.