How To Sew A French Seam - Step By Step! Place your fabric WRONG sides together and sew your seam with a quarter of an inch seam allowance. Trim your seam allowances by an eighth of an inch. Press the seam flat to set the stitches. Press the seam open. Now fold the fabric around the seam right sides together. Press the fold making sure its completely on the edge. Pin along that fold. And sew again, this time using a three eighths of an inch seam allowance. Then finally press