It's been years since I built one, so I don't recall exactly (Hell, I don't exactly recall what I had for breakfast), but there should be a faint line in the plastic to cut along. BTW, if you have a wife/girlfriend, this is a good time to put on the puppy-dog eyes. Women just seem to be really good at tedious little tasks like this (She probably even has a secret little pair of tiny scissors that she hides from you because... well, God only knows what we men would use them for).