Cannas multiply like rabbits, given ample water, and regular feeding. New stems should even now be coming up from the base. What area of the States are you in, Gumbyblooms? You can leave the roots in the ground, maybe with mulching, in Zones 8 and warmer. In colder areas, wait for the first frost, then cut off the frosted foliage and dig up the rhizomes. Shake the excess dirt off, and store them upside down in peat, in a chilly, frost-free area.
If you want to grow the seeds, wait until the spiny husks start to dry and open, then harvest the seeds, and keep them dry and cool until spring. You will need to file a nick in the hard seed coat to allow enough water to penetrate for the seeds to sprout. If your growing season is short, start indoors in a warm, sunny window a month or so before the last frost. Note that the seedlings could be very different in size and bloom color from their parent.