Justin Loretz shows you simple steps to removing your cassette.
Getting to the rear cassette for removal requires the rear wheel to come out of the frame. This job is easier if you can put the bike in a workstand, though it can be done when the bike is on the floor too.
Using the rear gear shifter place the chain onto the smallest cassette sprocket to make removal of the rear wheel easier. You can jam the rear derailleur as you try to remove the wheel if left in a larger sprocket.
Undo the rear quick release (QR). Depending on geometry of the rear drop-outs you may have to take a turn or two off the nut side to allow clean release from the frame. If the bike has a chain hook, use it.
With the QR undone, remove the wheel. Hold the rear derailleur body slightly rearwards and tap the top of the tyre with your other hand, this should allow the wheel to fall out of the rear drop-outs.
With the rear wheel now free from the frame you need to remove the QR lever assembly. This should just be a matter of unscrewing the nut end from the threaded section. Slide it out of the hub.
Get your cassette removal tool and insert it into the end of the hub on the cassette side. The splines on the tool should fit snugly inside the splines on the cassette lock ring. Sometimes they can need a wiggle.
Using the chain whip wrap the chain around one of the middle sprockets so force applied to the whip locks the cassette into drive. With the removal tool and chain whip in place apply counter rotational force.
Remove the tools and unwind the last threads of the lock ring before sliding the main body of the cassette off with your fingers. (Wear gloves to keep your hands clean.)