I wanted to make some headbands for the girls to go in their favor bags. I took some fold over elastic that I got from Sunshine Shoppe and I measured 15" for the girls. (The sunshine shoppe also has a list of measurements for newborns to adults found here.) I heat sealed all the edges by slowly passing a flame near the edges, to prevent fraying. Then I grabbed my trusty glue gun, put on a dab of glue and overlapped the edges.
I wanted the headband to be able to have accessories-flowers and other clips- that could be interchangeable so I took enough elastic to overlap my edges and I hot glued it to the inner part of the headband. This created a flap that will allow clips to be inserted in and there you have it.