I also fell in love with them too...especially since hydrangeas are and have ALWAYS been my favorite flower.
I use a lighter to fuse my FOE together. My sizes range from 12-17 (newborn to adult). I then take another small strip of foe to make an interchangeable loop..yes..I do it with my hydrangea headbands as well since I want to cover the seam of the headband.
I usually do three layers on my hydrangeas and glue them with hot glue right onto the headband. I accommodate for the stretch by putting the flowers fairly close together.
I also have bought most of mine from Michael's. My favorite ones are the "real feel" type hydrangeas. The pink and purple ones are those. They are amazing!!
I offer single, double and triple hydrangea headbands as well as clips and bobby pins. They are just too pretty!