Take a right amount of ribbon to tie around your ankle. I added a vertical piece to tie ribbon higher.
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.
Glue a satin ribbon rectangular to cover up stitching.
Voila! You're done :)