What type of yarn are you using?
Acrylic? Just wash it and dry it. Done.
Wool? Gentle wash. Lay out, pinning the center to length but leaving the ruffles loose. Some people put stuff (cotton, socks, fabric) in the folds to ruffle them but I think if you just fluff with your fingers creating a nice back-and-forth along the length and let it dry it will be okay.