my mistake, i think i misread some instructions i was given before when i was inquiring about Mugen pieces..i think the stock sideskirts you actually do have to take off..see if this helps you any:
Fitment on the VIS stuff is pretty decent. Ive scene a few that weren't so great, but they are usually pretty good. The stock rocker panels on the side need removed which is pretty easy. You should fit them before you paint them. On the sides, the best thing to do is buy a thin bar of aluminum, about 1.25 inches wide, 1/8 of inch thick, and 2-3 feet long. Use this bar to help keep a nice straight alignment in the door jam area. Line the bar, the top edge of the side skirt (lip), and the door jam. Pre-drill 3 wholes for screws. You need to have at least one other person help you with lining things up and holding. It will save you a lot of headache.
Here is a picture, you can see at the bottom the metal bar Im talking about, it creates a nice solid edge to screw against and will prevent the fiberglass lip from warping. You may be able to have the body shop have it molded into the skirt, but I just left it as is...
Then, underneath the car, you drill holes for where the bolts/plastic clips were on the original rocker panels. You should see on the fiberglass side skirt a little indented area that is a roughly points where the holes should go.
The rear is pretty easy. You leave the stock piece on, if you have a base though, it will need to be painted to match the car/rear apron. Line the piece up, then drill a couple holes for it inside the wheel well. Use existing locations of screws that currently hold the wheel well plastic shielding in.
Then you need to drill one more hole at the back. A screw should go up the bottom of the apron (in that small area that is about the same size as the rear lip piece) and then into the rear stock piece. That will hold the back of it.
Also, drill a couple holes into the bottom of the skirts to allow water to drain out, they tend to collect water easy and can get heavy causing them to sag.
If you have further questions, I can take some pics to show you better. But definitely work on it with someone.
lude4lifeh22 sent me those..helpful guy..you should ask him for specific help if you need any