Hold the hat in an inside grip high above the head. Lean the arm backwards and drop it behind your back to a catch on the heel.
For the catch it helps if you lift the toe before bending the knee. This makes sure the heel is nice and pointed. The only other hint I can give is to keep the legs together - this means that you can feel where the bent knee should be in relation to the leg that you are standing on. Just keep experimenting until you know where your heel should be.
Good luck ;o)