This will work best if the fabric is silk. I only used 6 circles for this flower because the smallest circle ended up being covered completely by the beads so I left it out. Make your largest circle as big as you want the finished flower to be. My flower is three inches wide.
A little trick about buying beads: Go to Claire's or Icing and buy a beaded bracelet or earrings off the clearance wall in the colors you need. Much, much cheaper than buying a big pack of way more beads than you need from a craft store.
If you prefer the flower to be permanently attached, then skip the pin and sew the flower directly to the hat. I'm all about options, so I prefer to keep the flower detachable in case I want to use it later as a lapel pin, hair accessory, or attached to a ribbon belt.