It was a while ago but here's a photo of what I took off mine when I fitted the fibreglass arches.
Measure around the top of the new arch to get the correct position and mark the holes, best the tape the new arch in position to do this.
In the instructions that came with my arches I think it said to measure 3 inches in from the outside of the new arch for the cut line. But that's only from memory, hopefully yours have their own instructions.
When I was cutting my rear arches I used a piece of wood clamped to the cut part of the arch to stop it from flapping around as I got to the top of the cut.
Oh, and make sure you align the new arch to the centre of the wheel as you only get one chance!!