First cut out the zip from the dess. You don't have to be that neat, but I like to keep zips for future projects.
Then I cut off the top of the dress. I pinned the line and cut an inch above the pins.
Then sew a hem (a running stitch is fine.)Make it wide enough for the elastic and safety pin to get through.
Attach a safety pin to the elastic and feed it through the hem, knot the elastic, hide the knot in the seam and sew up the gap.
And there you have it! All done!