What you will do:
Choose your ribbon. I chose this one that has small crystals because it compliments a lot of my kids current outfits. Also it has a textured back side so it won't slip. If you get slippery ribbon it won't stay in place.
Now use the ribbon to wrap around the "noggin" you will be putting it on, to see how long you will need it, keeping in mind that the elastic should be about 3", so cut the ribbon short 3"
I folded the ends of the ribbon to the insides about 1/4" (to make pretty, unfrayed ends) then attach the elastic to each end and stitch in place. Keep in mind if you choose ribbon with bling like this you don't want to sew over them, sew in between as shown. The elastic I used was white, I was playing around with fabric Sharpie's and colored it black. This shot also shows the textured side of the ribbon. I also used a little bit of hot glue to tack the ends of the elastic to the back side of the headband, just keeping things crisp and clean.
This is what it looks like from the side view.
This is a shot of the flower I made to go with it. It is a rosette style fabric flower made out of lace and accented with 3 pearls and a Swarovski crystal.
This is what the flower looks like from the back side. I used an alligator clip hot glued to attach it to the headband.