You could find a "boot cover" pattern at your local fabric store. Its a basic design that comes in most costume patterns. I know for a fact that the "Jedi" pattern from McCalls has one. Basically its a flap that wraps over the top of your footwear and has an elastic band that slips under the sneaker or boot at the instep and then goes a few inches up the shin and closes either with a strap at the top or zipper up the side. It sounds complicated but it is in fact not hard to make. You would then stitch said leather to the flap in whatever pattern you like. Choose a slightly heavier material if you want it to last.
Alternately, most costume shops sell this "boot cover" that I speak of either separately or in a pirate costume bag. They are normally made of some sort of pleather and are inexpensive. Same thing, just stitch your leather over it.
--Chazz