Your going to start with a favorite style of tank in a coordinating color to the fabric you plan on using for the skirt. You want to use a tank in your child's size (no need to size up or down) and. It the bottom of the tank off where you would want the waist of the skirt to be. I usually a measure from shoulder to belly button but you could go lower too to make a drop waist style or higher for an empire waist.
Next using 1/4" elastic cut a piece that is 1.75 times the width of the tank. Using the elastic here helps keep shape at the bottom of the tank so you won't see any stretching or pulling especially when it's a ribbed style as most tanks are.
Then stitch the cut ends together to make a circle and mark the 4 quarters (think 12, 3, 6, 9 on a clock. Do the same thing with your tank using the side seams as 2 of your marks. This helps ensure that you are evenly stretching the elastic as you go.
Using a stretch stitch (#3 on the my machine pictured) or a zig zag stitch if you don't have the other, attach the elastic to the very edge of the inside of the tank. You will want to stretch the elastic so that the quarter markings on the elastic match up to the pins on the tank. Be careful NOT to stretch the tank at the same time, that will defeat the purpose.
I didn't take pictures of the next few but here is where you will cute the skirt of the dress, I use 2 rectangles, about 30-36" x the desired length. I prefer a full skirt, but you wouldn't have to use as much if you didn't want to. It's easiest to hem your skirt here before you gather, so go ahead and hem and add any trim if desired.
Now, gather the skirt and pin right sides together (or clip... I love these little guys, Wonder Clips by Clover) and then either serge or use a zig zag stitch if you don't have a serger to attach the gathered skirt to the tank.
It's ok if you catch the edge of the elastic by accident since its really just there as a reinforcement and to stabilize the ribbed tank. The width of the elastic should just fit within the serged section and look like this...
Lastly, pull out the gathering stitches you used so that the tank and skirt can easily stretch.
That's it!! Now you can just sit back and enjoy (or make another one, be careful, they are addicting!)