welcome to FF guy. another wisconsinite!
i'd say start by cleaning the surface very well.
then peel an inch or two of the backing off the film and lay it on the brake light (lined up correctly)
use a credit card to squegee the air bubbles out and slowly pull the backing away and squegeeing the bubbles out.
after that, use a sharp blade to smoothly cut around the edge of the light.
then cut out the piece with the spray nozzle. then install and enjoy.