Grab a favorite necklace... the ones I have are long with solid links or beads, they are not adjustable. (If you have shorter necklace with clasps you can always open it up and add the rubber band there.)

You're going to need to play around with the size of the band you use to find one that will ensure a good fit. Gather up a variety.

Loop the necklace through the rubber band, once you've got it situated at the center of the necklace, fold the necklace over the rubber band....

....you should be able to fold the necklace over the rubber band and then spread the necklace out into a circle then....ta-da! You've got a necklace headband which will hopefully be the right size for your pretty little head. You can also use a ribbon instead of the band if you're feeling fancy. :-)