Pull out your original front and back dress pattern pieces. You will need to reshape them so they go straight down, instead of angling out into an a-line. Measure your dress-wearer from neckline to dropped waist (about where their pants usually sit), and crop the dress off at that length, making sure to add seam allowances. In this illustration, the dotted line shows how you will be cutting your new front and back pattern pieces:
Using your new front and back bodice pieces as a guide, cut a strip of fabric that is 3 inches long and equal in width to the bottom hem of your new bodice, like so:
Sew those two strips together on a short end, forming one long strip. Now, depending on how full you want your skirt, cut a rectangle of fabric for your skirt. For the length, measure your dress-wearer from 2 inches below the dropped waist to wherever you want the skirt hem to fall. Add 2 inches for the seam allowance and hem. (Or use fabric like I used in the example, which is already hemmed for you!) The width is entirely up to you, but I suggest making it 1.5 to 2 times the width of your long strip for a nice gather. First, if necessary, hem the bottom of your skirt rectangle by turning it up a half inch, and then turning it up another inch and edge stitching. Then gather the top using a basting stitch and pulling the bobbin threads until the top of the rectangle is the same width as your long strip, like so:
Right sides together, sew the skirt to the strip. Finish the inside seam by serging, zig-zag stitching, or pinking.
Open up your bodice so it lays flat, and sew the long strip to the edge of the bodice. Finish the inside seam.
Now that the skirt is attached, flip the garment inside out and sew the other side, from sleeve hem to skirt hem, making sure to line up your seams as you go. Finish this seam.