I know this already. I've made sheaths in the past for various knives of mine. When I make them, I line the sheath with bulk sinew that I have so it doesnt come in direct contact with the leather. I mainly use a sinew lining to protect the leather from being cut. When I make it, I soak the sinew for close to 24hrs, I coat the blade with a "No Salt" cooking spray to avoid rust while in the drying process, clamp the sinew down to that, and put it over a heat vent to dry. Once it's dried out, I remove the clamps and glue the leather to the sinew and clamp it down again until dried. Once the glue is set, I trim the sinew around the blade, I punch small holes through the edge of the leather & sinew and use a cross stitch to lace it up. Then and only then will I cut rest of the excess sinew and sew the rest of the leather to form.