Twin-stitch neckline working from the right side (double line of stitching is visible on outside.)
Tips: Start somewhere inconspicuous on the back neckline. To finish, simply sew exactly over the top of your stitching for about 2.5cm(1"). To keep stitching straight, line up edge of foot with fold of fabric. No need to stretch as you sew as the zigzag stitch formed on the back gives your seam added stretch.
On the inside, trim seam edges neatly, close to the zigzag stitch.