Nylon thread would probably do ok. Use a thimble because it will probably be hard to push the need through the leather and patch. Stick the needle from behind and through the patch and it should hide the knot. You could spray the back of the patch with 505 adhesive spray and temporarily adhere the patch to the leather so it won't move around while you sew it on, but 505 if kind of expensive. It won't be fast to hand sew them on just so you know.