I found it much easier to remove the rear bumper cover so that I could accurately drill the holes for attachment. You must be
very careful to align properly so the seams line up near the exhaust. I also found it a PITA to reinstall the side bolts from the top inner bumper panel, into the diffuser threaded inserts.
BTW, I purchased mine unpainted and did the finish work myself with paint from
www.paintscratch.com Several coats of base, then clear coat and buff.