Position your pencil in your left hand and the blade in your right. Keep the blade stationary and *pull* the pencil against the blade by pushing away from it with your left thumb (see video). This is more stable than pushing the blade against a stationary pencil, which is more likely to break the charcoal/graphite.
Slowly work around the graphite or charcoal, removing the wooden barrel a sliver at a time in long, slow motions.
I generally aim for about 1/2 inch to 3/4 inch of exposed charcoal and 1/2 inch to 1 inch of graphite.