Warning, once you have divided your clump, it will take two to three years before it flowers again. They actually like to be pot bound and flower best in big clumps. So perhaps you will re-think the division. If you are worried about the clump encroaching on a wall you could "root prune" it by digging all around the clump so that the rhizomes will be severed. In order to divide the clump, you would need to dig it up and separate the individual plants. Make sure that each plant has several leaves along with the roots. You may have to use a sharp knife to separate the plants.