I'm assuming the bumper is pre-drilled for a lip?
If not, let me know and I'll see if I can find that DIY on drilling it.
Otherwise you need part number 51112252200 (maybe 10 of them?). They're small white pieces that you put into the holes in the bumper and then turn to lock in place. Then you put the lip on and screw some bolts through the holes in the bottom of the lip into those white pieces. The bolts are number 51112257140.
You also need a couple of expanding rivets for the sides underneath the lip (# 51111908077).
Edit: Here's that DIY in case you need it: