Step 6: Pin together and stitch
You will have the two little elastic tails sticking out on one short side; the rest of the loop should be sandwiched between the two pieces. If you have never sewn before this might be odd to you, but just trust me, this is correct. :)
Now, stitch around three of the sides with a 1/4" seam allowance, leaving the short side without the elastic un-stitched. I used a serger with a 4-thread overlock, which has two rows of stitches going around the edge and is nice and strong.
If you are using a conventional sewing machine or sewing by hand, you should add another row of stitches along the short edge with the elastic to help secure it in place.
If you are hand-stitching, I recommend a backstitch or something similar for the best results; it is sturdier than a running stitch.