Every custom sheath starts with the blade, not the leather. Length, width at the guard, spine thickness. Photos with a ruler in frame work well, or for a perfect fit the knife itself can be sent in and returned wearing its new sheath.
The leather is full-grain vegetable-tanned hide, cut oversize, soaked, and wet-moulded around the actual blade shape so the sheath clicks home with no rattle. Tooling (basketweave, floral, skulls, your own design) happens while the leather is cased and takes the most time of any step.
After dyeing and assembly comes saddle stitching by hand: two needles, one thread, every hole. It is slower than a machine and it is also why the seam does not unravel if a stitch ever wears through. A final burnish on the edges and it is ready for its knife.