There's more than one way of making a ruffle skirt, and here's the method I used:
First, cut out your skirt piece - decide on your skirt length; and for the width, take your waist measurement and double it.
Make a casing at the top of the skirt by folding the top ruffle to the back and catching it with your stitches.
Cut a piece of 1/4" elastic to your waist size and feed it through the casing with a safety pin attached.
Secure the ends of the elastic together with a double zig-zag stitch.
Match your two unfinished edges of the skirt and sew them right-sides together - take your time and make sure to have all the ruffles facing down by either matching them together as you sew or you could pin them all before you start sewing.
Here you can see the finished seam.
And that's it! So simple, and such a fun fabric to play with. My daughter says it's just fancy enough to go with her new top. And I have just enough left over ruffle fabric to make a newborn size one for my new little niece . . .