Hey YIN, Nice job on the sheath, Your sheath may be a little thick in the one area but that is also a big knife so a bold sheath and welt is not out of line. When I'm doing something thick like that i start out with my 4 prong punch through the first layer. That establishes the angle for the awl. then i drill if required with a 1/16" bit. (I think you can get numbered bits that are smaller but many bench drill presses will only tighten down on a 1/16 bit unless you change the chuck.) Now you can push your waxed awl through following the angle of the punch, when you sew you get a nice looking stitch, after that i use a roller stitch marker to go over the threads that helps to sink the thread into the grove and evens out the stitches.
For fine work like you are doing you want a drill press as this keeps the holes plum as long as you shim up the tapered end when drilling. You can also use the nail system like someone else mentioned find one the right size you are good to go. Good luck on the next one your customer will be happy with this one.