First cut the ribbon if it's one long piece.
Put the ribbon right under the arch.
Mark where the ribbon goes.
Either hand or machine baste.
Try the shoes on and make sure the ribbon has been placed properly. If so, sew with smaller stitches to secure. I stitch three times across to really secure the ribbon. Basting makes it easier to undo if the ribbon was not placed properly.
After all that remove the elastic. The ribbons will hold the ballet flat in place while dancing. Happy dancing, y'all.