Making ruffles with your sewing machine is a skill that is useful for constructing curtains, little girls' dresses, throw pillows, and more. While the traditional method shown in sewing books is to sew two parallel rows of basting, then pull the ends to gather the ruffle (hoping the threads don't break and necessitate starting over) there is a better way. It is not difficult, involving only a straight stitch and a zig-zag stitch.
First, cut the ruffle fabric according to your pattern. If you are not using a pattern, measure the length of the area to which the ruffle will be sewn; hemline, pillow perimeter, or curtain width. The length of the ruffle piece should be two to four times the desired finished length of the ruffle. The width of the ruffle is the desired finished width plus a seam allowance at the top and a hem at the bottom. Piece several rectangles together if you need a long piece.
Hem the narrow ends of the ruffle, unless the ends will be sewn to each other (as with a pillow or a skirt hem.) Hem one long edge as well. Leave the other long edge raw as that will be the gathering edge.
Lay a long piece of crochet cotton ( the gathering string) parallel to and about 3/8 inch from the raw edge. Feed the fabric and the string under the sewing machine's presser foot so that the string is directly in the path of the needle if a straight stitch is sewn. Set the machine for a medium zig-zag stitch, checking to be sure the needle will swing in such a way that the gathering string is not caught in the stitches, but is encased by the zig-zags. Be sure the stitches will all be inside the seam allowance. Stitch the length of the ruffle.
Secure one end of the crochet cotton to the ruffle with a pin to make sure it will not be pulled out of its zig-zag casing. Grasp the other end of the string and pull until the ruffle is gathered to the correct length. Secure the loose end of crochet cotton with another pin. Baste (with pins or stitches) the ruffle to the edge you are ruffling (the "base fabric"), right sides together, making sure the gathers are distributed evenly along the entire length of the ruffle. Sew the ruffle and the base fabric together using a regular straight stitch. Remove the gathering string by unpinning the ends and pulling on one end.
This method is much more satisfactory than the parallel basting stitches method for most applications. If, however, you have many long ruffles to make, e.g. making ruffled curtains for your entire house, a ruffler attachment is available for most sewing machines. Making ruffles with your sewing machine is a simple way to add a special touch to your sewing projects, and this method will give good results on most fabrics.