sewing. 4. Start sewing! You can use either a straight stitch or zigzag stitch. Begin at the hem, securing the end with a stitch and backstitch. Then sew carefully in a straight line while stretching the elastic up to the end point of your gathering. Secure at the top with a backstitch, then snip the threads and the extra elastic. Et voil! You can now stitch the underarm seam and hem and attach the sleeve to your garment.