Seedpods can be left to develop on the plant.
Notice how some of the seedpods are open are ready to pop out and disperse themselves in the garden. Seeds can be left to drop from the seedpod naturally. But if you don’t want lots of little plants popping up everywhere it is a good idea to cut of the seed heads and save the seeds. I also like to snip off the seedpods to tidy up the appearance of the plants.
When you can snip off the seedpods, shake out the small black seeds into a container. The seeds can be planted in the spring (slightly under the ground). You can always start the seeds inside in small pots about 6 weeks before it is time to plant outside.
You can take softwood stem cuttings when the plant starts growing in the spring. Cut a stem with an angle cut, remove any buds, and dip in rooting compound before putting into a pot. Be sure the stem has a few nodes on it, and get a couple of these below the surface of the dirt as this is where the root develops from.
I find the best and easiest way to get more plants is to divide the original plant. Dig the clump and pull apart into a few smaller plants and then replant in the new location. Be sure to get some of the root system as the plant tend to spread its stems but it doesn’t always root.