One way to gather is to have a long bobbin thread when starting. Needs to be the same length or longer as the fabric length you are ruffling. Bring that bobbin thread up over the top of the fabric and lay it the length of the fabric toward the direction you are sewing. Put down the presser foot. Using a loose zig zag stitch (adjust your stitch length-I use a 2) and a width of 4-6, sew over the bobbin thread. Leave a long bobbin thread tail at the end. Pull the bobbin thread and gather up the fabric. Done!