There is a special tool which is like a solid steel bar which bolts onto the flywheel - in its centre there is a threaded screw which bears directly onto the crankshaft. However you can usually get the flywheel off by undoing the crankshaft nut a few turns, then gently prising the flywheel up with 2 screwdrivers....a sharp rap with a hammer onto the slack crankshaft nut usually springs the flywheel loose.
John, February 2008