A long time ago, my father worked in a head shop in the late 60s early 70s. He did custom leather work, vests, bags, various adornments, etc. His old leather-working tools are still in that basement, and I have picked up a little of the craft. Here is an ible on how to make a custom wrist cuff.
Leather (3"x9-10", at least 2mm thick)
hole punch (any awl will do in a pinch, but a leather hole punch is ideal)
leather rivet and snap setting kit
small leather rivets
The first step is to decide the circumference, measure your wrist and decide how much slack you want.
Mark with a pencil on the leather where you want to cut. Using razor blade, cut along a straight-edge until you have the desired shape and size.
Step 1: Holes!
Wrap leather around your wrist and mark on the overlapped section where you want your snaps to be.
Line up a ruler with the marks for the snaps (I had already done the snaps in the photo) and mark approximately 3/4 inch intervals for even spacing.
Use your hole-punch to make holes slightly larger than the rivet studs. Repeat on the other side.