Are you using pre-made rosettes? Or do you actually need to make them, too?
Here's one website w/instructions
You can probably google to find more.
I found one hat w/instructions said to make the ties 6" long. That seems kind of short to me, though. Double it if you want the ribbon uncut, I'd add a few inches - maybe 15" for the ribbon tie part, plus however much you need for the 2 rosettes.
Use regular sewing thread to sew it to the hat.