Like most throing moves, this is a variation on the Throw to head - so it's probably a good idea to learn that first!
Hold the hat in your right hand using an inside grip, spread your fingers out nice and wide as you would for a normal throw to the head.
Swing your arm out to the side, so that the crown of the hat is pointing downwards. Bring your arm behind your back and release the hat so that it does a nice, high, slow spin above your left shoulder.
As the hat descends, look up into it and catch it in the same way that you
would for a standard throw to the head.
Top Tip: The catching part is easier if you turn your head towards your left
shoulder just before the catch. Oh, and obviously learn this throw from both