There are many youtube video actually teaches how to make a watch straps. But my suggestion is that, before learning how to make a watch strap, u can actually practice on something easier like a simple credit card wallet.
From my experience, making a watch straps is alot more harder than making a credit card wallet due to more sophisticated process. To be more accurate, u need more precise measuring and precise cutting for making a watch strap. That is why a watch strap actually cost more than a handmade leather wallet. This is just my 2 cent, i could be wrong.
Here are some credit card wallet that i actually done this few weeks.