Recently I posted in the wrist exercise thread, but it will be great if I could add a routine for pronation supination into my workouts as well. So, what is the 'wrist curl' of pronation and supination?
One of the best methods is to use an old grip handle. Tie a sturdy string around it, and hang a 2L pop bottle at the bottom, filled with half to 1 L water, depending on your strength. Then supinate/pronate your hands around the handle to bring the bottle slowly up and then slowly down repeatedly, while concentrating the use of your racket hand.
Can you describe how to supinate/pronate around the handle? I'd think it's not the typical exercise where you use both hands to roll up the bottle. It sounds like a one arm exercise where supinating/pronating the handle will result in the handle going this direction:
If you want your forearm to be stronger, I would suggest multi-joint exercises. Multijoint exercises allow you to work build muscle on different parts of your body. Examples = Squat, Deadlift, Shoulder Press, Bench Press, and Pull ups. You will get a better workout for your whole body rather than just your "wrist".