Cut a length of ribbon, use your head and the hair tie to measure. Cut the ribbon longer than necessary so you can trim excess.
Fold and iron a loop of ribbon around the hair tie at one end. Sew and back stitch along the length of the hair tie.
Measure one last time by safety pinning other end of ribbon around hair tie. Try the headband on and make adjustments as needed.
Sew along second loop, same as first.
Variations of ribbon headband to try: braided ribbons, beaded ribbons, sewing a smaller ribbon on top of wider ribbon, adding embellishments of all kinds.