Do not throw the ball hard, medium speed is fine. Release the ball a bit like you would put a shot. That is, do not snap the wrist. When done properly, there is no dominant point of wind resistance on the ball as with a fastball or curve ball. Hence, the ball will dance up and down, in and out.
Unpredictable movement is the main characteristic of a knuckle ball.
See this URL for photos of the Tom Candiotti grip of the knuckle ball as well as those of several other knuckleball pitchers.