First, unlock the two halves of the cassette, this can be accomplished in two ways:

Using two chain whips with the wheel off the bike

Using one chain whip with the wheel on the bike

If using the two chain whip method, put one chain whip on each half of the cassette.

Orient the chain whip on the aluminum portion of the cassette to turn the cassette clockwise.

Orient the chain whip on the steel portion of the cassette to turn counter-clockwise.

Turn the chain whips until the arrow on the steel sprocket assembly reaches the unlock arrow.

If only using one chain whip, leave the wheel on the bike and shift the chain into one of the aluminum sprockets.

Put the chain whip on the steel portion of the cassette, orient it to turn the steel sprocket assembly counter-clockwise.

While holding tension on the chain by using the cranks, turn the chainwhip counter clockwise until the arrow on the steel sprocket assembly lines up with the unlock arrow on the aluminum piece.

Once you have turned the cassette to the unlock position, remove the steel sprocket assembly from the hub.

Put the chain whip on the aluminum piece, orient it to pull the sprocket clockwise.

Using an e*thriteen Bottom Bracket (BB) tool, loosen the cassette lockring.

Remove the cassette lockring, then remove the aluminum cassette piece.

If you have the newer pinch-bolt style cassette. Use a T25 to loosen the pinch-bolt on the collar.