Follow the line on both sides of the windshield with masking tape. Get the good stuff too and leave the tape on until you're completely finished. We used a jigsaw with a fine blade, working very slowly so's not to melt too much of the plastic. Have an extra pair of hands ready so that as you get to the end of the cut, it doesn't snap off and ruin anything. Once cut, we used sandpaper to smooth the edge and get rid of any saw marks. Go finer and finer in grit until you're at around 1000-1200. At that point, you can either hit it with some polish to get it to look like the rest of the edge.
My friend actually made a VERY fast swipe over the edge with a torch. Someone had told him that it puts a factory-molded-looking gloss back to the edge and it seemed to work.
