First, cover your headband with ribbon. I used a hot glue gun but you can also use Fabritac - a versatile fabric glue that works on anything. Glue one color first, then the second color if you're using two strands of ribbon.
You don't have to tuck it in around the sides because when it's on the head, you won't see anything but the ribbon. Just tuck the ribbon at the bottoms of the headband like you see in the picture.
Then, with whatever ribbon you're using, secure everything in a circle with your glue.
With a small piece, wrap it around the front of the bow and tighten it in place in the back. Glue one end first and then wrap tightly the other piece around and secure everything down with your glue.
Here's the front:
Glue the bow on the headband and you're done!
**NOTE: For a baby, instead of wrapping a plastic headband, you'd have to use some stretchy, lacy elastic. Measure the baby's head, giving plenty of slack because you don't want it too tight. Glue the elastic together and glue the bow right where you pieced the elastic together to hide your edges.