The heels are measured around the ankle. They want to overlap the point where the uppers connect to the sole by about an inch on each side.
The heels are traced onto the hide, cut out, and waxed. The heel pattern is a curved rectangle. The convex portion (the lower part in the pictures) becomes the bottom of the heel, and is sewn to the sole. The edges of the top part of the heel are rounded off slightly.
Note that here the top heel is longer than the bottom heel. Rather than measure one and then duplicate it, each heel was fitted to the foot. The mocs may not look exactly alike when done, but they will fit comfortably.
The heels are darted onto the sole.