TricksWithHats
Hold the hat by the brim in an outside grip with the crown facing forwards.
Swing your arm up infront of you, until your hand is about eye level, with the hat tilting slightly upwards. If you let go at the top of the swing, the hat's momentum should carry it through, and it will roll down your arm.
The movement that follows is similar to the one arm tumble. Look down your arm, tuck your chin in, and raise your arm slightly so that the hat rolls downhill and on to your head.
Once you can do that, try throwing the hat from your left hand, and using the right hand to guide it into a dynamic arm tumble.