1. Using 2 different fabrics, cut out 5 6-inch squares from each fabric for a total of 10 6-inch squares. If the fabric is thin/lightweight, use interfacing for best results. (interfacing is the stuff that you iron or sew to fabric to make it more stiff).
2. Outer shell: Sew 4 of your 6-inch squares together using a 1/4 inch seam allowance as pictured below.
3. Sew the 2 ends of your row together to make an unfinished cube.
Things I would have done differently: Used stiffer interfacing or stiffer cardboard, ironed the fabric before sewing it together, and I wish I had known a good way to sew up that final gap. It was awkward and difficult and doesn't look very good. But it worked I guess. Also, I accidently trimmed my cardboard pieces a little too small, so the fabric isn't as tight as it should be.