Holes are punched into the soles with an awl. The holes are spaced about 3/8″ apart.
Artificial sinew is sewn in and out the holes, starting at the heel and winding around the front, and back again to the heel.
The sole is fitted around the foot. Before fitting, the thread is pulled tight, and the hide bunched up at the toes and heel so that it looks like it will roughly fit the foot. Then it’s put on the foot, and the bunched-up parts are fine tuned for a good fit. Finally the thread is pulled tight and tied off.