Hope this tutorial is helpful; leave a comment if you have questions.
Step 1: Cut out your fabric
You'll also need a sewing machine.
(I also let my daughter decorate the fabric to make blocks for her cousin--this step is optional ;)
Step 2: Sew the first 2 pieces together
Ok, so to do this, stack your pieces of fabric good sides together with the edges lined up. You can see in the photo that I drew a line to show exactly where I needed to sew (this is just for the tutorial--you don't need to do this). Plant the needle 1/4 inch in from the edge and take a few stitches. Back stitch to lock the threads and continue stitching all the way across the project.
1/4 inch from the end, plant your needle and back stitch again.
Step 3: Attach the third piece of fabric
When you're lining up the fabric, again, put good sides together. Then line up the edges of 2 pieces and fold the seam allowance of the third piece out of the way (you can do this because you left 1/4 in at the end of the seam). Look at the photo, this is hard to describe.
When stitching this seam, once again start and stop 1/4 inch from each end of the fabric and back stitch to lock the stitches. (see the second photo)
Step 4: Sew the last seam
Step 5: Turn your corner right side out (plus bonus content)
If you wish to make a cube, continue adding squares to create a cube shape (it will be inside out). When you're ready to sew the last seam, stitch the corners and leave a gap along the center of the edge so you can turn the cube right side out (see third photo). You'll have to hand stitch closed the opening.