1. Using a contrasting thread colour, do two parallel rows of stitches with longest possible stitch length. If your seam allowance is 1 cm (3/8”), do the first stitch 5 mm (1/4”) from the edge, and the second stitch 1,5 cm (1/2”) from the edge. This way the stitches will end up one on each side of the seam line. If you’re gathering long pieces such as an entire skirt, do separate stitches for back and front pieces. Backstitch at one end and leave a long tail of thread at the other end. Don’t cheat by doing just one line of gathering stitches! Your gathers will look much better with two lines of stitches, and they will stay in place better when sewing over the top of the gathers.
5. Stitch the two layers together between the two gathering stitches. (Remember to change stitch length back to normal, unless you want to baste first) Finish the raw edge with your preferred method.
Give it a quick press and you're all done!