I'm assuming you mean the ruffles at the bottom of the white underskirt? You have a few options:
- Purchase pre-ruffled trim (available at Jo-Ann's, Hancocks, etc.)
- Use a ruffler foot. Start with a strip of fabric that is already hemmed. The width should be the length you want your ruffle to be, plus a seam allowance. A ruffler foot will automatically tuck and sew your fabric as you feed it through, and they are usually adjustable (dense ruffles or sparse ruffles).
- If you don't have a ruffler foot, or don't wish to purchase one, you can gather a ruffle by hand, too. Using the longest stitch length on your machine, do a quick running stitch all the way down your pre-hemmed strip of fabric. Flip your fabric over. Pull gently on the underside thread to gather your fabric. You'll have to work your way down the length of the fabric carefully, making sure not to break your thread.
- And last, but not least, make sure you start with a strip of fabric that is significantly longer than the length you want the finished ruffle to be.
Alternatively, if you're referring more to the skirts (I suppose they're like really big ruffles), I'd suggest doing them as circle skirts. The more circles you use (I like to use at least two full circles), the fuller the skirt will be.