What tool do you guys use and what's the best and easiest way?

Use a high speed cutting wheel device of some sort as to not flake the paint. If you're trying to preserve the paint, painters tape can act as a stencil and help keep the paint from chipping and flaking.

Good idea and no it needs a new paint job gonna leave it unpainted till I get it done and go hurt some feelings then I'll get it painted

I measured and put painter's tape around the edges. Then I scribed a cut line. Then I went to town with a Dremel and cut-off reinforced wheel. Didn't take too long. I slapped the core on the car and test fitted it before I cleaned up the edges.