A friend of mine did this on his wrx and since it came out pretty clean I decided to do it myself. All you need is a garage door bottom seal from your local home depot which goes for about $8.67 for 9ft, and of course some screws. Double sided tape is optional.
The Garage Door Bottom Seal I purchased looked like this:
This is a before shot with the main ingredient laying on the ground:
Align and stretch the seal along the bottom of your bumper to see where to cut the excess. Unfortunately, being that I suffer from chronic laziness I did not measure it for you, I apologize. You'll soon learn that you must cut around some plastic which is on your bumper to have the seal sit correctly. The following pic shows the simple cut I made to get around the plastic:
Now simply start carefully screwing the seal to your bumper where you see fit. If you choose to use double sided tape this is where you'd need it to help the seal stick b4 you start poking holes through your bumper.
Thats it. After shots:
