If you are experiencing RSI in your wrist you need to restrict movement there. Similar to using a mouse this will be caused from twisting, bending and stretching your wrist.
You need to try and eliminate as much movement as you can. When writing and drawing try and move with your arm, keeping your wrist straight. This will alleviate the pressure on your wrist and the symptoms of RSI.
Your posture and seating position is also important. Position yourself so that your wrist is not bent up or down. Your wrist in your natural writing position should be flat. Bending will add pressure.
It would be an extreme but you can get braces to keep wrist in a constant position. It may be advisable to try this to train yourself to write without moving your wrist.
I hope this has helped and I'm happy to answer any of your questions