When I make a sheath for a knife, I just use the knife as a pattern. My methods would probably be considered unorthodox though. Somebody should chime in with some help.
Use the knife, and make you a pattern out of heavy paper, then take it apart and use the pattern to cut you one out of leather. stitch and decorate it to please yourself. JMHO
remember to give yourself about a half inch clearance between the edge of the knife and the edge (seams) of the sheath. If you plan on pushing the handle into the sheath also, give a little more room where it will be as when the handle pushes the leather out to the front and back to squeeze in, it will draw in the sides.
You might want to use a welt? I think it's called. A spacer between the front and back piece to make room for the blade and hopefully keep it off the stitches.
You can use one piece and wrap it around with just one seam down the blade edge side. You can use a seperate piece for each side, and sew down the edge on both sides. You can use a welt or not. You can cut a belt loop in the back or sew one on. Look at some web sites or catalogues to get an idea of what you want, or go to the Hobbies forum here and search knives to see examples. I think I posted a pic of several sheaths I made there, and you can see where I made one the same width all the way down, then when I shoved the knife in there it made the sheath look narrower at the top.