It depends what tool you are using.
TABLET/ PEN/CINTIQ- Hold the stylus/pen like you would a normal pencil and draw with your elbow. Rest you hand on the tablet but try to keep the motion coming from your elbow, this will prevent carpal tunnel syndrome. Your hand, fingers, elbow, and arm are all pivots. If you naturally swing you arm with a pencil and draw, you will make a perfect circle. The same with your elbow, hold it in place and you will draw a perfect semi circle, and so will your wrist. But you have to hold it steady and draw confidently.
Google - Davinci's vitruvian man. Davinci knew this and his drawing illustrates this concept.