When doing a ruffle I increase my stitch length to large stitches and sew along about 1/4 inch from the edge of the fabric strip edge. I then tie the thread ends together on one end. On the other edge, I pull the bobbin thread and the strip can be gathered into a ruffle. I tie the thread end together when the ruffle is the right size to fit my project. I then pin the ruffle in place and sew it on at regular stitch length.