Use a hobby knife you will more than likely screw it up. Too easy to slip. I have seen it and done it pleanty.

Go to the hobby store that sells rc car stuff, get a pair of lexan scissors, less than $5, and use them. You get a very clean cut, very precise and you don't slip and cut the heck out of the canopy or your fingers.

Trust me, you use a hobby knife, you will screw up as many canopies as you do successfully in most cases. There are a few that can do them well, but they are the exception.