Make sure you're sitting high enough for the workstation to be slightly below elbow height, so that your hand rests naturally on the mouse. Let it relax. Hold it loosely in your hand, relaxing your shoulder and grip. Resist the tendency to stiffen your wrist. Experiment with different ways to move the mouse.
Some people drag the mouse, moving the cursor in a continuous line.
Some lift the mouse slightly, and brush it against the mouse pad with small strokes. Others prefer to slide it, loosely tossing the mouse between the thumb and the ring finger. Try whatever feels right for you, but make sure the mouse is set at a speed that's comfortable for your hand and eyes. Consider the shape and size of the mouse, how comfortably it fits into your hand, and any special features that might make your job easier. Try not to rely on any rigid rules of job and workstation design.