I found this awesome tutorial over at Our Nerd Home and absolutely had to make it! I would list instructions here, but they outlined it better. You need black foamcore that is the appropriate size for the terrifying gaping fireplace hole that you wish to cover (like I did), a way to cut perfect squares (either a punch or a sliding paper cutter), appropriately (or inappropriately!) colored paper, a glue stick, and a lot of free time. Have fun!