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The elastic should be long enough to attach at the back of the heel, run over your ankle, and then reattach at the back of the heel on the other side. The ends of the elastic should attach one finger's width away from the back heel seam on either side. You can position the elastic on the inside or outside of the shoe. If you want to attach your elastic to the outside of your shoe, position it low, near where the heel meets the floor. Elastics sewn on the inside should be positioned all the way down in the shoe. Keep in mind that elastic sewn on the inside of the shoe may irritate your skin. Once you have positioned your elastic, pin it in place and slip your foot into the shoe to make sure it fits properly. Be careful not to poke your foot! The elastic should be snug, but not so tight that it cuts off your circulation. The elastic should be tilted slightly forward so that it lies flat against your ankle. Sew each end of your elastic onto the shoe using dental floss or reinforced thread. Start your stitching near the top of the shoe, just under the drawstring. Continue stitching along each side of the elastic end, so that your stitching forms a box. Be sure that you don't stitch over the drawstring. Only stitch through the canvas lining of the shoe. You don't want the stitches to show on the silk outer layer! You'll need your shoes to be even, of course! Sew elastic onto your other shoe following the steps outlined above.
Measure the elastic. Position the elastic. Pin and test your elastic. Sew the elastic ends onto the shoe. Repeat the process on your other shoe.