I used a utility knife but it takes a while. I have seen a piece of steel tubing with the end ground sharp used. This worked real well especially when heated, but it takes a while to make the tool. Someone actually sells this tool, but not real cheap.
Others have used a hole saw which only works if you do not plan to clip it.
If you only have one to do the utility knife works fine, just be careful to not cut too far especially if your fingers are in the way. You can take a torch and melt the fibers if you want.