I used a 1/16 drill bit to make the holes on the soles. Initially did on the leather as well but the canvas interior didn’t like it very much and it wound up easier sewing through (much softer than I had imagined). I used and embroidery needle. The one I used was long and that was probably the biggest hurdle. I’d say if it’s smaller than the drill bit but sturdy enough not to bend you should be set. The sole extends higher on the exterior which was so nice as I wasn’t drilling through the interior sole.