There's a tool that makes flat holes, lined up at a slant. Looks like a little pitchfork with one to six prongs. They are mainly used to sew with flat leather lacing but they can be got very small (I got mine on auction for cheap). 3mm size is about right for sinew type flat thread, the elongated hole makes it much easier to keep the thread facing one way and once you get it working it looks amazingly pro. Just search for prong hole punch or look up lacing stitching on youtube, same principle only smaller, if you can find the tool in black metal it lasts better than the silver and makes better holes, carbon steel or whatever it is. You could possibly learn the idea faster with a larger set and using lacing. The idea is a round hole for round lacing and a flat hole for flat lacing. In general I find *not twisted or braided thread is much easier to make look good (imitation sinew).