1. Measure where a headband would sit around your head (roughly from ear to ear plus a couple of inches)
2. Cut ribbon or fabric to that length
3. Cut a small piece of elastic (about half the measurement of the gap the ribbon doesn't cover)
4. Fold the edges of one end of the cut ribbon around the elastic and zig-zag stitch the elastic in place.
5. Repeat step 4 with other end of ribbon, forming a circle.
6. Try on the headband, and adjust the fit as necessary (ex: mine was too big, so I folded the elastic over a bit and sewed it smaller)
To add little something extra to this, you could put a bow on it.