Grosgrain Ribbon (here)... Try white or oyster.
1. Tie a bow with the grosgrain ribbon. I stumbled upon this great tutorial on how to tie a variety of bows (here). So many adorable styles to choose from!
2. In order to keep the cut edge from fraying, you can add fray check (here) along the edge, or simply roll the edges two times and sew with a straight stitch.
3. Then, add a little dab of glue, either e-6000 or a hot glue gun, and glue onto the sandal. Have fun!!