The horology Crazy blog has a good tutorial on how to make modifications to regular watch straps. Sometimes you'll find that the band is a little too long for your taste, and on rubber straps it just takes a little work to get a good custom length. This guide shows how it's done on a Seiko diver. There's lots of photos of the entire process and the proper tools required for the job.

Link

Before modification of the strap end

After the mod