as with all 3M installs, the more care you take during the installation will result in a better fit and lasting finish.
some tips i used to install both my ARKYM rear lip (previous) and the replica GTS-V lip:
1) make sure both surfaces are clean and free from grease/ oil and DRY, i didn't use any surface prep before hand, just soap and dry
2) test fit on your bumper to see where the diffuser touches the bumper to know where to put the 3M tape. especially for the replica diffuser, this may not be at the outer most corners of the lip
3) line the diffuser with 3M tape and press down firmly
4) mount on your bumper with OEM screws (not tightened)
5) once you got the diffuser where you want it, slowly remove the 3M tape from one end to the next.
6) use masking tape or painters tape (liberally) to ensure upward pressure for at least 24hrs!
to me point 6 is the most important step. Good luck with your installs!