Shimano gear clusters thread on to a shaft or "spline" that extends from the centre of the back wheel axle. To remove a Shimano cluster or "cassette," you need a simple tool called a chain-whip. The chain whip wraps around the gears. It has a small handle that you push on to loosen the cassette from the spline. You can remove a gear cluster in less than 15 minutes.
Remove the wheel from the bike. Unscrew the lever from the axle and pull out the skewer. Reach around the wheel with your other hand and catch the knob and spring when you pull out the skewer. Set them aside.
Screw the lock-ring remover into the threads that surround the divot where the skewer came out.
Bend over and stand the wheel up against your knees. With your left hand, thread the chain whip over the gear cluster from left to right with the handle on your left side facing down.
Place the wrench on the lock-ring remover on the right side of the cluster opposite the chain-whip.
Push down on the chain whip clockwise, while you push down on the wrench counterclockwise. When the gear cluster loosens, take off the wrench, chain-whip and lock-ring remover and finish unscrewing the gear cluster with your fingers.