Dude, just use a straight edge to make your marks with a Sharpie, then carefully score the fenders on your marks with a utility knife. Go over it a few times and you'll be able to break the plastic clean. Out does such a good job that when you're done you'll only need a piece of sandpaper wrapped around a block of wood to touch up the edges.
I found that the carbide cutting wheel for the dremel worked best. I tryed using the black cutting wheels, and they melted the plastic more than they cut it. The carbide wheel cut nice, and you can use a lower RPM on the dremel. It was also way easier to control.
I finished out my cuts with 60 grit on a orbital sander to round the edges, then got progressive finer.
I did a lot of cutting on mine. ALOT!! lol
I think it turned out pretty damn nice.