Is this a good product to use when cutting out squares to fit each window pane? Do you just add soapy water and then run a credit card or squidgy over it? Or if not, what product do you recommend for

This film would work great for this application. You can easily pre-cut the squares when installing. For full installation instructions visit: https://www.decorativefilm.com/installation-instructions. We also have a wide variety of individual colors that can be used as well. You can find those here: https://www.decorativefilm.com/colored-films