It was a fairly large hole....I think about 3/8" - so you want to be sure of your location before you take the drill to the wall
The well-nuts themselves are a brass nut surrounded by rubber, about 1/2" deep. Most of the well-nut slides into the 3/8" hole with a small lip that rests on the interior (visible side) of the shell skin. You then place the mount over the hole and insert the screw. The screw needs be be long enough to catch the threads on the far side of the well-nut. As you turn, the rubber compresses against the other side of shell skin making a solid mount that won't pull thru the holes and because of the rubber on both sides of the skin, it is somewhat buffered from shock/vibration.
I had to get the proper threaded screws from Home Depot - the Walmart mount only came with wood screws with plastic wall anchors.