I ran across the idea for this on Wild Olive, and had to try it for myself.
Seriously easy, all I did was cut the tank top up the front in the middle, you could just leave it like that and it would be cute. I decided I wanted to add ruffles, so I measured how long the edge I just cut was (mine was 21") and I multiplied that number by 1 1/2 and added a few more inches, so I cut two pieces of fabric that were 2 1/4" wide by 35" long, then I serged the long edges, sewed a basting stitch down the middle and gathered it up, (here's a video on you tube that shows you how to make a gather) and sewed it on, turning under the short edges sewing those down. If you don't have a serger, you could zig zag the edges, or even leave them and they will fray a bit, which would be cool too. Hopefully that made sense, it really is easy.