So where to begin.... A base is needed this can be from buckram, sinamay, vilene depends on whether you want the piece as a trim or a fully fledged hat.
Take a strip and create it into a bias strip by folding both outside edges into the centre this can be pressed if wish. Then begin creating the cabouchon rose centre. Lay the strip at a 3.00 o'clock line on the base and pin in place. Then take a 2nd strip and do the same but at half past nine, interlock the two pieces at the top, pin in place.
Take some more strips but this time just fold them in half, create three about 4 inches long and lay them around the centre of the flower, as in the picture, pin in place. You can at this point tack them in place or just keep adding strips and pinning them in place and tack right at the end.
Your rose should be looking something a little like this. There are a range of variations you could twist the fabric out a little to create petal like edges, you could add other fabrics or patterns to create different looks.
The flower on the left shows it all still pinned in place. the one on the right tacked down. Now for the final layer. This can be done with one long bias strip or three more pieces as before to follow the pattern.
Lay the fabric with the right side facing the right side of the rose, pin and stitch in place with running stitch, then turn the fabric over to the underside, this hides all stitched and gives a polished finish.
On the left is the underside of the rose, the fabric can then be slip stitched in place and if you wish you can line it. the finished piece is on the right. The rose could be attached onto a comb or onto a hat itself.