Basically, follow the Coop voting thread. When you see something you want, follow the topic for it in Seed Runs Co-op - Overgrow.com. The signup does not usually open until the seeds are ready and they sometimes fill up quickly. So you have to stay on top of it. Then twice a year a “box” goes out with the seeds you signed up for. You pay a small amount of money to cover shipping costs.
Instructions are here: How do I sign up for a co-op Seed Run