Step 1: Pull Out Clumps
Cut down old stems so that you can see the crown. Dig around the clump and pry it out with a fork. Shake off surplus soil, and wash out any weed roots. Try teasing apart the clump by hand. If this is too hard, insert two forks back to back and work the handles together to split the clump. Repeat until you have enough good-sized segments.
Step 2: Replant Segments
Replant segments in fresh, well-prepared ground before the roots have a chance to dry out. Check that the plants are at the same depth as before, firm in and level the soil, and water well.
Division is the easiest method of making new plants and is the usual way of increasing most herbaceous perennials.