Scissors are the worst way if you want clean edges imo.

The best way is to score it with a razor, then snap all the extra off.

I always start with a nice sharp utility knife or exacto blade and go over it real carefully, then go over it once again as some of the spots will need to be a little deeper to get them to break cleanly.

Then just clean up the wheel wells and corners with a sanding drum and that's about it.