The non-permanent window films that stick on with water are really easy to apply and work well. All you do is clean the window well, then cut a piece of film about an inch larger than the window in both directions, then roll it up and start applying it from the top down. Use a spray bottle to wet the window. Leaving a half-inch overlap (to cut off when you're done) onto the frame, smooth the top edge across the full width of the window. Use a squeegee to smooth the film from the centre out to the sides. Work your way down to the bottom until the film is all smoothed. Use an exacto knife to cut off the excess around the frame, getting right into the crease so the glass is completely covered.

We've had some up in the school where I worked for over a year and a half, and unless the students pick at it, it's still as good as the day it was installed. They did pick off a corner and I found it was fairly easy to pull off the damaged piece and install a new piece. If the windows are large, you will need someone to assist.