I sew A LOT of ruffles for the aprons I sell on Etsy. If you have ever sewn ruffles, you will know that it can be pretty time consuming mostly because the ruffles are usually sewn from about three times the length of fabric that you want the ruffle to be.
Sewing ruffles is very easy. All you do is set your straight stitch to its longest length and sew 1/4 inch from one of the edges. Then sew again along the same edge 1/4 inch next to the first row of stitches. Make sure that you leave the threads longs at the beginning and end of each row of stitches, and do NOT backstitch. All that is left to do is pull on both top threads at one of the ends of the rows of stitches to gather up the fabric into your ruffles. Easy!
But like I mentioned before this can be very time consuming, especially if you are making long lengths of ruffles. One easy way I have found to cut the time in half is to sew both rows of stitches at once using a double needle. I can't really take credit for this idea because my mom was the one who suggested it :) But I really think it is a great idea!! (Maybe it is obvious to most people and everyone actually does this already?)
Using the double needle you make your ruffle pretty much the same way as above, except this time you are only sewing the edge once. You will end up with two rows of stitches on the top, and still only one bobbin thread on the bottom. When I first tried this, I was a little nervous that it wouldn't work because of the single bobbin thread. However, when you pull on the bobbin thread you will see that the fabric gathers up just as easily as when you sew the two rows separately!