I got this idea and the instructions from
Crazy Mom Quilts. She has lots of amazing things on her blog!
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.
4. Pin the 5th square to the bottom of your cube and sew with a 1/4 inch seam. Reapeat the process for the inner shell,
except when sewing the bottom square, only sew 3 of the 4 seams. You will use this gap for turning and placing your cardboard inserts. You should now have 2 topless boxes.
5. Once your inner shell is complete place it inside your outer shell with the "right" sides together. Pin and sew around the entire perimiter of the cube using a 1/4 inch seam allowance.
6. Through your gap, turn the cube right side out and sew around the perimiter once again using a very small seam allowance. Pictured below.
7. Using any type of cardboard box (I used cereal boxes), cut 5 sqaures each measuring about 5 1/8 inches. Don't use any cardboard that has a fold in it. It won't stand straight once your finished.
8. Through the gap wrestle the cardboard into each side and the bottom of your cube.
9. Carefully sew your gap (I have no suggestions on the easiest way of doing this), making sure the cardboard doesn't slip out of place and you're finished.
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.