Prepare for sewing by double-threading your needle. Double-thread the needle by inserting the thread through the hole in the needle, leaving an equal length of thread, roughly 12 to 16 inches long, on both sides of the eye of the needle. Tie a small knot at the end of the thread, using both ends.
Slip on your ballet shoes. Before you begin to sew elastics, you must adjust the ballet shoes' drawstrings to a comfortable fit. Pull drawstring tight for a snug fit, but not so snug that it cuts off circulation or is uncomfortable. Shoe should easily slip on and off with drawstring tied. Tie drawstring in a bow when adjusted.
With ballet shoes still on, mark the placement and measure the length for your elastic. For ankle placement, flex your foot, and place the elastic on both sides of the back seam, about 1/8 to 1/4 inch away from the heel seam. For arch placement, place elastic just to the back (towards the heel) of the side seam on your shoes. For both methods, make light pencil marks along both sides of elastic on the inside of shoe. To measure length, stretch the elastic so that it is taut, but still comfortable. You will need to put your whole foot through the elastic, and it should not cut into your ankle when wearing. Cut off excess elastic, remembering to leave about 1/4 inch extra on both sides for sewing.
Take off ballet shoes, and hold the elastic in place according to your pencil marks. You should have about 1/4 inch of the elastic placed inside the shoe. Whip stitch, short, ¼ inch stitches, around both sides and bottom of the elastic. To whip stitch, start on the top right side of the elastic and insert the needle through the canvas lining inside the shoe, close to your elastic. Bring the needle up through the elastic, about a quarter inch from where you originally inserted the needle. Repeat this stitch until you have sewn around the right side, bottom and left side of the elastic. Do not sew across the top of the elastic.
When you have reached the top left side of the elastic, knot the thread by slipping the needle under several competed stitches, and pulling the thread tight. Knot the thread several times for security. Cut any excess thread close to the knot. Repeat Steps 4 and 5 for the other end of the elastic.
Repeat Steps 2 through 5 for your second ballet shoe.