I had done cakes that had rows of fondant that looked like pleats before, but they all started at the bottom and worked their way up. This one was more ragged-looking and irregular at the top edge, and it looked like the best way to do it would be to use gumpaste to get the thin edge that you'd need.
I didn't want to cover an entire cake with rows of gumpaste, but I also needed to get the ruffled edge thin enough that it did look like fabric, but would also keep its shape.
I decided to use a 50/50 fondant/modeling chocolate combination, since I've done that a bunch of times to cover cakes to make the fondant taste better and give it some extra body. The chocolate in the fondant would also let me roll it out thinner without ripping it.
You can do this with an ombre effect, or with alternating colors of ruffles. Wider bands will give you fewer rows of ruffles, but will allow you to cover the cake faster.
For a point of reference, this was a 6-9-12" cake, and it took me 3 1/2 hours to do all the ruffles once the cake was assembled and iced. The 6" tier took an hour, but I think that I got faster as I got the routine down.