Put some masking tape around the outside, which can serve 2 purposes... one to mark where you want to cut, and two to make sure you don't chip your bumper.
To finish up the job, I just used some clearcoat... sprayed it in the lid of a can, and applied to the cut edge with a small paint brush. It has worked pretty good, although I didn't cut it enough, and it wound up melting the bumper a little on the top. You can't see it unless you stare at it though.
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