When my dad and I installed my Scopione lip, we used mounting screws for the holes that matched up with the pre-existing ones in the bottom of the bumper (I think there were 6 screws). We then used several plastic anchor snaps for the other holes that are designed to mount the front bumper of a GMC conversion van.
When my dad was laying under the car, after he put the last anchor in, he grabbed the lip with his hand and lifted himself off the ground. The lip didn't budge. Needless to say, I don't think that lip is going anywhere.