actually, stock sideskirts are made of the same material and stock front and rear bumpers. Just wetsand with 400 grit, and the after your finished, make sure you bondo all the little chips and stuff. painted skirts show nics and damage alot more then unpainted ones.

primer, then paint. unless your like white or black, red, I don't recommend doing them with walmart paint. You can try but it might waste your time. Milano Red is usually easy to come by...and white and black are a given...

but just get a bodyshop to do it, and have them add the flexing agent they add to the paint when doing the front and rear bumpers, they'll know what your talking about.