Fold the heel of the shoe in toward the toe so it is flat against the insole. The folded flat part of the shoe should resemble a triangle.
Place the elastic strap across the widest part of the triangle you made by folding down the heel cup.
Thread your needle and stitch in one end of the elastic with the cut end of the elastic pointing down toward the sole of the shoe. Keep the thread doubled and stitch through only the lining of the shoe if you can. Sew down one side of the cut end of the elastic, across the raw end, and up the other side until you reach the binding. Cut the thread and double knot the ends. Sew underneath and not through the drawstring --- the drawstring should still be able to move freely when you are finished.
Put on the shoe and pull the elastic across the top of your foot to just under your ankle bone on the opposite side. The elastic should be snug but not so tight that it is uncomfortable or cuts off circulation. Mark the elastic with a pencil at the point where it would be sewn when stretched to the appropriate position. Release the elastic and remove the shoe.
Cut the elastic at the pencil mark using your scissors. Fold the heel cup in flat again to locate the final position for the other end of the elastic. Sew the other end of the elastic to the shoe with the cut end facing the insole, again being careful not to sew across the drawstring.