If you take a piece of paper that is square then cut it on the diagonal twice you have the 45 degree triangle for the kaleidoscope. You can use this as a pattern to take to a store and have it cut (be sure and tell them that you need the exact size as your template). To get the size you need, your square needs to be twice as wide as the size of triangle you need is tall. If you only need to use it a few times, glue it to some of thea template plastic and cut the template plastic with a "paper" rotary blade (you don't want to cut fabric with it after using it on the pastic so I use an old retired blade).