Got some stretch knit to work with? Once you have this base bodice, you can mix up the parts below in any sort of skirt you like. Empire waist? Gathered segments? An aline design with trim?
1. Trace around an existing tank shirt adding a little room for seam allowance.
2. Cut two fronts and two backs. Cut the neckline of the front a little deeper than the back portion.
3. Right sides together, sew the two fronts to each other, stretching gently. Leave the bottom open.
4. Do the same thing to the two back pieces so that when you turn them right side out, you have one nicely lined front and one nicely lined back. Press with an iron. I don't recommend topstitching unless you s-t-r-e-t-c-h a lot as you go (otherwise you end up with a taut stitch line that won't stretch to accomodate your head).
Now layer the front piece over the back piece (right sides together) and line up the shoulder seams. Stitch. Do the same with the side seams.
Here I added a typical gathered skirt, then layered on a few extras. The bottom of the skirt was hemmed and I added an elastic to give it the bubble effect.
I added a black jersey band at the waist. On top of that band, I ruffled some black/white jersey and stitched it on.
The front of the skirt is a random series of strips set on the diagonal. This is quick and easy to do with knits because they don't fray.
A downside the the stretch factor - the weight of my add-ons is pulling down the front of the single layer skirt - see how it's shorter at the sides? Not that my four year old cares a whiff. She lubs it.