I cut down a screen from my previous bike.
I put masking tape along the line of the cut, and drew the line of the cut on the masking tape. I then cut it slowly and carefully with a disc cutting wheel on a Dremel. Once cut, I sanded and shaped the cut edge with finer and finer sandpaper to shape and round off the edge, and I finished with a very fine wet and dry. It turned out a very neat job indeed, with no cut marks showing and a neat rounded edge.