Fender flares are very easy to trim. Mark it, tape it, make a guide, whatever you prefer, just remember the "measure twice, cut once" rule!
When you cut them, just score it with a razor / box cutter / utility knife. Do that a few times to get a decent groove, then fold it back & forth and it will pop apart. Sand the edge smooth. I used a paint stripping wheel in my drill to radius the inside corners evenly, but I'm too lazy to take the time & measure LoL!
If they're already painted & chipping, you might want to think about sanding them down & repainting when you're done trimming.