did you drill the bottom sections where there are the screws that attach the plastic engine splash guards to the bumper?
The lip clearly goes over them

 

I didnt, I've never needed to access those screws. you planning on taking your bumper completely off at some point? ive opened my headlights a few times which requires taking the bumper most of the way off, but i never needed to access those screws on the bottom that are covered up

 

If you pull on the lip you can still access the screws. It's a pain, but that's what i did when i was working on my headlights. Or just use a right angle bit.

 

I just wanted to close this up for those who still might have a question about this, but pretty much what mredenberg said above the lip doesnt sit flush under the bumper so you can deff access any rivets or screws after the install

 

i modded my stillen lip to my bumper best way to go pics will be up soon