Whilst many ballet shoes now come with elastics attached, these are not always suitable for high insteps, or anything other than standard width feet.
Once removed, or if there is no elastic attached, it can be difficult to know where to stitch them.
The tried and tested method is to fold down the back of the shoe and mark where the back meets the side. This is where the elastic or ribbon should be stitched. Make sure it is slanted slightly at an angle forward towards the toe of the shoe. Stitch carefully ensuring you don't sew through any of the drawcord around the edge.