I brought this STI lip
Subaru Impreza WRX 2006 2007 STI S204 Front Bumper Lip Spoiler Bodykit | eBay . I installed initially using the screws that came with it then changed them out for thicker screws. I started by centering the lip and using a drilling with Philips bit drilled the first one in. Also used a wide washer. From the center I worked outwards on both sides bending the lip until it fit the bumper. Once I got to the end I used a dremel to cut the ends at an angle to make it flow with the bumper. I did not use double sided tape. Also the first time I used ramps but difficult to drive back down. Second time used jack stands.
Hope this helps.