I have never done the flattened bottle caps, but I do the regular bottle caps on my hair bows. I have just been gluing the ribbon down to the back of the bottle cap (with E6000) and when that dries I add a wooden disk over the ribbon (also with the E600) and then after that dries use the E600 to glue to the center of the bow as well.
I actually just got a bow from a friend who used a button for the center (with a shank on the back). She threaded the ribbon (that you use to wrap around the center of the bow) through the hole on the shank and then put a small amount of glue on
the button to keep from sliding down the ribbon and then attached to the bow. I thought this was a great idea for bottle caps and then you dont have to wait for the E600 glue to dry during each step of the ribbon, wood disk, etc. You can buy plain shankes at www.buttonshanks.com.