Using your fingers to help make your final trim is an easy way to be sure you are cutting the pattern to its proper size. You'll notice when placing your rough-cut pattern on the window, the frame tends to hold the film away from the gasket. Making your trim without laying the film down to the gasket can cause you to end up with a larger gap than desired. Here are a couple of different ways of using your fingers to make your final trim.
Option 1) When making your final trim, use the fingers on your free hand to lead your knife. Basically, this means holding the knife with your right hand and using your left index finger to make the film lay flat against the glass and gasket (or vice versa if you are left handed). Follow your finger with your knife, making for a good close trim.
Option 2) Use the fingers on your cutting hand (one-handed technique). Holding your knife with your thumb, index finger, and middle finger, take your free ring and pinkie fingers to guide the knife. The easiest way is to use your pinkie to lay the film down while making the trim.
These two techniques using your fingers are very easy to perfect with just a little practice. They will also help make your cuts much cleaner and closer.