Use two pieces (or four) with the stitching needle in the middle of your length of synthetic sinew. Since there is two layers of leather I pry it apart at the very bottom and introduce the needle there with at knot at the very end. Pull snug and bury the knot. Start lacing going in the top of the sheath, skipping the first hole, and come back underneath towards yourself to the hole you skipped, separate the sinew into two strands and pass the needle and sinew through. Repeat until you get to the top of the sheath. At the top you need to finish underneath and then just feed the needle through each stitch on the bottom, snugging each tight. The end result is a super strong V stitch that won't let go even if cut.
Does that make sense?