I ran two screws in the front fender, a single self tapping screw on the rear fender and at one point I ran two big screws up through the random small brackets on the underside of the car. I also ran double sided tape on the two flat parts of the skirts that meet the fenders.
In Japan they're held one with a series of clips like US kouki side skirts. You would have to drill holes in the body of your car and buy the 8 dollar a piece clips (I think there are 8 on each side). You can see the holes in the side of my car:
http://farm3.static.flickr.com/2501/...c27613f7_b.jpg
The yellow things are part of the clip, they slide in and out so some stuck with the side skirts when I took them off.
A zip tie up front and back and some double sided tape will be perfectly fine though.