What they said. Attach the rail to bike. Lay hump over top of it to mark your first cut. Cut off less then you originally think is safe. You can always cut off more,but cant put back what is already missing. I had 2-4 trips back and forth to the bike for trial fittings. Light sand on the cut corners. Not on the painted finish.
Not hard at all,just take your time. Dremel will melt the plastic as it cut it. Careful you don't get hit in eye and such with flying hot plastic. It's not going to just fling all over. maybe not at all. Just a heads up. Glasses is a great safety thing to have.