You will need:
Small amount light worsted weight yarn (I used Caron Simply Soft)
About 15 pony beads (I didn't use that many that's just to be on the safe side)
Thread the 15 beads onto the yarn
Attach the yarn to the hair elastic with a slip stitch ch1, sc onto elastic band, *ch 2, slide bead up next to hook and ch around it, ch1, 2 sc onto band, repeat from * until band is full, make sure it's completely covered but stretch the band to add more, end with* ch 2, slide bead up next to hook and ch around it, ch1* Join to first sc and end. Work in the ends and you're done.
I used 12 and 13 beads in the ones above, they were different size bands. How many beads you will use depends on the size of the elastic. It's better to have more beads ready then not enough.