There are a couple of ways to make more of the ferns in your garden. You can wait for them to grow (some spread faster than others) and divide them. Or collect and sprout their spores.
Spores are like little seeds, though they're much smaller and slower to germinate and grow. They're found on the fern fronds instead of a seed pod, capsule, or fruit. Spores appear as little bumps, often black or brown, lining the underside of some fronds.