I had those flats that were slipping off my feet ... so I added a cotton ribbon to hold them in place. It's supereasy.
1
Take a right amount of ribbon to tie around your ankle. I added a vertical piece to tie ribbon higher.
2
Sew it to back of your flats - not in the middle of ribbon - make sure that the knot will be on the outer side of anke after tying.
Don't forget to treat ends of ribbon with fire to secure it from unpicking.
3
Glue a satin ribbon rectangular to cover up stitching.
Voila! You're done :)