So I used some of the pre-elaticized fabric being sold now. I bought mine at JoAnn's with a 40% off coupon. All I did was buy a cut of fabric 1" smaller than my daughter's chest measurement (it's sold by the inch), trim it more neatly at home, measure for the length (17" below the last elastic line for a 9 year-old), sew the back seam and hem (using a narrow hem foot--that will be the next post). I think it took me a total of 1 1/2 hours. I spent some extra time fixing where some the elastic came undone and then reinforced the seam over the elastic with a really short stitch length so that I didn't have to worry about the elastic coming undone again. This dress could have been even quicker if I hadn't bought such a long fabric. I had to hem it.
But the fabric is really cute, isn't it? My girl has discovered that she loves paisley.
On a down note, after her first day of wearing it, the elastic irritated her skin so I'll need to add a liner of some stretchy t-shirt material to the front of the elastic part.