Pin two pieces of the fabric together with the "wrong" sides facing in and the "right" sides facing out. The "wrong" side is the side you do not want to be visible in the final product. Pin along the entire length of the seam.
Sew a seam with a 1/4 inch allowance along the length of the pinned edge. Cut the seam allowance to 1/8 inch from the stitches.
Unfold the fabric, and lay it flat with the right side facing up. Iron the length of the seam, pressing both edges flat to the same side. Fold the fabric along the seam with the wrong sides facing out. Iron the seam flat at the fold on the wrong side.
Pin the fabric together along the seam. This time the wrong sides should be facing out.
Sew a seam with a 3/8 inch allowance along the entire length of the pinned edge. Remove the pins. Unfold the fabric with the wrong sides up, and iron the seam flat.