I'm not sure if you mean just the cassette body or the entire side-plate. Here's how to get the side-plate off:
Ideally, you need to make a tool to unscrew the side-plate so that you can get some leverage. You turn the side-plate using the holes that the the three countersunk screws screw into, so you need to remove one or more of the screws. If you can't fabricate a lever tool, you can knock it round using a single cap head screw with a washer under the head to keep it out of the countersink and to protect the side-plate. A large washer with a small hole is best if you have one. If using the single screw method, draw a pencil line across the join near where the spokes are so that you can see when the side-plate moves, and take out all three screws to release the clutch on the inside, which will reduce the side-plate's inertia. Screw the screw down moderately tight, then whack the head in the direction of sideplate rotation (counterclockwise). After a few blows, it might start to bend, so use a new screw in a new hole. Keep watching the pencil line to see any progress. When the pencil lines have noved more than 20mm, you can probably unscrew it by hand.
Whichever method you use, it'll be a lot easier if you heat up the hub between the spokes with a hot air gun until it's just about too hot to touch.
You can make the lever tool out of 2mm steel sheet or thicker. Dremmel out a holea bit bigger than the boss in the middle of the side-plate, then mark and drill the three holes for holes, then cut the outside shape. Alternatively you can use a widish strip of slightly thicker steel and grind out clearance for the boss with an angle grinder, then mark and drill two holes where the screw-holes are.
If you mean to remove the cassette spline from the side-plate, you need to unscrew the ring nut on the inside of the side-plate. IIRC, it's dogged, so you might need to fabricate a dogged tool. Maybe use an angle grinder to grind the shape in the edge of a normal 1/2" drive socket.
