I molded, glued, and used clinch nails on the sole. nailed from the bottom of the sole upwards. When you use clinch nails, the nails are in a continuous taper. When you nail against something hard (like a cobbler's last or an anvil) the tip of the nail curls back on itself and back into the upper sole. That way, even if the nail head wears out, the taper of the nail will keep the sole attached. I didn't sew the soles because I don't like that look for sandals. In my mind, molded sandals don't look good sewn. Making them wasn't too hard but I had to find the right nails, figure out the correct way to fit the sandals so they were comfortable, and figure out how to cut the sole leather. Those were a learning experience but I'm glad I made them.
Andrew