There's really only one way to mount those feet. Drill holes where you want them, put the screws through, and tighten.

The hole you drill should be slightly larger than the screw itself, but not so big that the foot can flop around. It looks like you can take a screw out of the foot to compare with your drill bit. Mark your locations with a center punch (or a nail) before you start drilling to keep the bit from wandering and you should be good to go. The hardest part is making sure you can get to the nut on the inside to tighten it up.