Measure out a length of 1/2″ or 3/4″ elastic–for an adult, this will likely be the adult’s waist measurement + 1″; for a child, who doesn’t have hips, this will likely be more snug–their exact waist measurement, perhaps.
Fasten a safety pin to one end of the elastic, and feed it through the waistband casing. Overlap the ends of the elastic, sew them together very sturdily, then push this last piece of elastic back into the casing and sew closed that last open section of casing.
Once sewn, the petticoat is ready to wear as-is. If the tulle is especially scratchy, however, you may want to sew a slip to be worn underneath the petticoat, or use any leftover lining from the Prom dress to sew an underskirt directly to the waistband of the petticoat.
To wear the petticoat, simply slip it on underneath the formal dress. For extra security, as I did when my daughter modeled her Rainbow Fairy Dress in our town’s big Trashion/Refashion Show, you can also hand-tack the waistband of the petticoat, using just a few hand-sewn stitches, to the inside of the dress at the correct spot for the petticoat to sit.