Slipstitch along the folded edge from the outside. On the inside fold back the pressed edge and machine stitch along the seam line. Use double-faced basting tape. Place the tape along the folded edge of the seam allowance and match stripes. As you stitch be careful not to stitch through the tape.
Sergers feed the fabric evenly, matching stripes as the stitching is done.
Just begin with stripes matched. Some sewing machines have an attachment called an even-feed pressure foot. It moves the top fabric at the same speed as the underneath feed dog. If you start with the stripes matching, it will continue to stay matched for the length of the seam, with some pinning. Stripes can add a new dimension to anyone's wardrobe.