For anyone who's interested, it's a very simple design that I can take absolutely no credit for (but hey, they probably nicked it from some other designer anyway). Here's how I did it:
I used one of R's skirts to work out how long I wanted the skirt to be (in my case, 25cm) and figured the wide waistband would compensate for the need to add seam allowances. So I cut about 25cm of satin, using the entire width of the piece for the underskirt. I sewed the back seam together, creating a loop of fabric, zigzagged the top edge and hemmed the bottom edge. I did much the same for the tulle but cut it so it would end up a little longer than the satin underskirt, and also folded it to make a double layer. Obviously that part didn't need hemming or finishing (hooray). The tulle was wider than the satin so after I sewed the back seam I gathered it a bit to make it the same width, then sewed the tulle and satin together around the top edge with a basting stitch. Then it was a simple matter of sewing the ends of the elastic together to form a waistband loop, pinning the skirt to the inside of the elastic (right side of skirt to wrong side of elastic), stretching the elastic out and sewing. Then unpicking because I failed to catch portions of the skirt while sewing. Then resewing. And unpicking again. Tearing my hair out while cursing my own incompetence. But finally getting there.
I finished it off with a satin flower from this tutorial, using black beads for the centre of the flower. I glued the whole thing to a little badge so it would be removable. Ta da! I tried it on R only to see it fall down - the waistband was too big. Oh dear. Unpick. Resew.
But it looks very sweet on her - and she even likes it.