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Using a handheld knife will take forever. Plus, your cuts won't be as nice as it could be. On my messed up bumper I had, I used a box cutter blade and it wasn't that fun.
If you have access to a Dremel (wired with at least 20,000 rpm speed), it would make things much much easier and faster for you. Make sure you use the proper attachment tool (high speed cutter). I said wired because battery runs out faster than you think and 10,000 is too damn slow so higher the speed the better.