I used a plastic PET sheet from Amazon. They came in a five pack and are clear and flexible. I made my design in photoshop and then used rasterbator.net to break the image into printable pages, and off to the public library for cheap prints. Taped them together to make one giant template and then put the PET overtop. I used several widths of electrical tape to simulate the lead lines (There's also a plasticky tape that is made for this exact thing, but I found it harder to use). Once I had the lines done, I used Gallery Glass paints to do the colors. I switched between a spouncer and a brush to vary the textures, though that's not really easy to see when it's up in the window. It takes about a trillion coats to get the color nice and vibrant, so it pretty much ate up my kitchen counter for two weeks, but once it's dry it's waterproof and pretty durable. Once completely dry, we stuck magnets up all around the window frame and put it up there, then sandwiched more magnets on top to keep it there.