Simple installation, simply pull the old one out, and insert your new cartridge and you're away.

You should notice that on 95% of old pistons that the washers have water blisters.This stops the washer from properly seating on the base, therefore causing a leaking cistern. 

Note: the old piston that you are taking out may be a very dark green and pink. Do not be alarmed as this is normal and the new white one will be its replacement.