Step 2: Construct the sides
Turn one right-side out and place it inside the other so the right sides are together. It's tidy but not essential to match up the seams. Pin if you like, and stitch around the long edge.
Turn this right-side out now, so the seam is folded to the inside. I find it helpful to stitch the other long edge together at this point. This time the wrong sides are together.