Take the bumper off, so that you have access to the existing holes on the bottom of the headlight buckets/extensions. Position the airdam where you expect it to be when mounted, with the lip at the top of the airdam behind the headlight bucket. Mark and drill two holes, using the existing holes in the bottom of the headlight bucket as reference. You'll need to drill one more hole at the back of the airdam, where it lines up with the bottom of fender. Get some good nuts, bolts, flat washers, and lock washers and mount it back up. Don't overtighten the bolts. Once you get it mounted back up, reinstall the bumper and you'll be good to go.
Below is a pic for reference. The position of the bolt locations I used are noted in red (you cant see the back bolt, since it's on the back side sandwiching the fender and airdam together).