Don't try to install window film using just a credit card and soapy water – you'll ruin the finish. I highly recommend buying the proper professional installation tools which Tintfit sells directly to the public here: https://www.tintfit.com/ For a few pounds, you can get the correct squeegees, scraper blades, and application sprays that the trade professionals use. Having the right kit makes the difference between a bubbly DIY job and a perfect, invisible finish.