Remember the Duck, Duck Goose shirt? Here it is with it's ruffle.
The ruffles are really easy. Just cut a two-inch wides strip to about 48 inches long (or use 4 foot long strips, which is what I did.) I used the selvage end of the fabric so I wouldn't have to hem it. Then ruffle your strip. I found a very easy way to get even gathers is to extend you stitch length to the longest setting, and turn the tension on your machine all the way up, and it will gather it for you.
To add a little bit more pizazz, I sewed bias tape along the gathered edge. (This also hides any uneven gathering.) Then just sew the whole ruffle to the shirt, and join the two ends of the ruffle. (I lined mine up with the side seam of the shirt, and you can't see it at all.)
And there are my mini circle skirts! What do you think?