Using a spade, dig up the whole plant. Then use your hands or a knife to break it into smaller pieces, each with some leaves and roots. Replant the new divisions in areas that have the same light & soil conditions as the original plant. Water the new plants well, and if it's really hot and sunny, provide them with some shade from an umbrella or an overturned laundry basket for a few days until they get established in their new spot.
Division does not work well for plants that have one long tap root such as baptisia, balloon flower, butterfly weed or oriental poppy.