You have to wait for them to ripen. This won't happen until the fall. When the seedheads and their stems are completely browned out, and the grass itself is starting to fade, the seeds will just about fall off in your hand. Using scissors, cut off the seedhead. Lay them out on paper plates inside for a few weeks, to dry, and then separate the individual seeds from the seedheads. If you want to go further, you can remove the actual seeds from their hulls. I do this, so I know I have mature and viable seeds. Even with thoroughly aged seedheads, some hulls contain no seed, or shriveled un-viable seed.

In your zone, you probably won't have enough growing season left to sow them in the fall. I use winter sowing techniques to sow mine, and have found that they don't need cold stratification in order to germinate. The later I sow them, the better germination I get. I sow them in containers of potting mix, and keep them in a sunny spot, keeping them moist. Lightly cover them (no more than 1/16th" of mix) and keep the covering mix moist until they germinate.

If you want to direct sow them, wait until after your last frost, and do the same thing. Cover them very lightly with soil, and keep that covering moist until they germinate.

They're super easy to grow from seed.

Thank you, donn! They self-sow so easily that I knew they must be easy to grow from seed but I'd not a clue as to how to do it.