Nate,
I mark out out the width of the strip with a pencil and then use a metal spirit level (or any stright edge will do) and a utility knife and lightly score the first cut. A second score usually gets through the plastic, but if not, just bend and snap. Make sure to cut the corners off at 45 degrees at each end, so the bottom would look like this \___/ for authenticity
Now hurry up and get the kit finished!