Thread the yarn needle with the end of the elastic drawstring as you would thread a needle with thread.
Hold a slipper in your hand and examine the slipper at the front, top, inner edge. You should see a small opening in the casing where you'll insert the tip of the needle to rethread the drawstring through the casing.
Insert the needle carefully into the hole and begin passing the needle along the casing of the ballet slipper.
Move the needle along the casing with your fingers until the needle goes all the way around the top edge of the ballet slipper and emerges from the same hole.
Pull the ends of the elastic drawstring to make them even in the ballet slipper.
Cut a 12-inch length of thread and thread the hand-sewing needle. Tie a knot in the ends of the thread. Stitch one or two small stitches through the elastic drawstring in at the back edge of the ballet slipper to secure the drawstring in the casing and prevent it from coming out again. Tie a knot and clip off the excess thread.
Repeat the process with the other ballet slipper (if necessary).