Measure out the width you want the ruffle and double it - then cut. You will need to make sure that the material you cut is a lot longer than the width of your outfit since you want it to ruffle. Put your machine on the longest stitch and sew the edges together. BE SURE AN DON'T BACK & FORWARD STITCH BECAUSE YOU WILL BE PULLING THE THREADS. Start out by making the hanging strings longer than normal since you are going to pull them. Sew all the way to the end of the fabric and cut the strings longer. DON'T BACK AND FORWARD STITCH ON THE OTHER END EITHER! Take one end of the ruffle and find the string that will pull through the fabric the easiest. Pull it little and then move the material along to ruffle it. Do this the whole length of your fabric. Be careful not to pull to hard b/c strings can break and you will have to start all over. When you get your ruffle how you want it, pin to the garment and sew on.
I do my ruffles a little different. I do measure the garment hem or wherever for the length I need. So if my skirt measures 40 inches I cut enough for 80. I sew the ends of the ruffle together and make a big loop. I pin my 2 edges(seams) to match the garment, then I pin only the center and 2 or three places on either side of the center line. This way when I'm sewing I just gather it as I go along
RUFFLES - I do the same idea kind of but I use dental floss. Sew it on with zigzag, going over the floss, then pull the floss at the end. Pin and sew.
I have a Gathering "trick" that produces the most beautiful gathers and ruffles. I use 3 gathering threads, one on the seam, one above and one below. That way, after you gather it up you have 2 holding rows keeping your gathers just perfect as you sew. This keeps the garment looking "handmade" instead of "homemade" It does take an extra minute or so, but it looks better than any other way. They turn out perfect.