As the season starts to wind down I have been letting my lettuce bolt and flower. To get seeds you have to let the flower mature and turn into what looks like a dandelion ball. You pull the pods free and separate the seeds.
The seeds are really small, but each seed pod will have 15 or more. I have saved a lot because I wanted to start some new plants to get baby greens now and for next year. When I plant next year, I plan on planting more successively. So every week or two I will start some new plants. Some plants won't sprout in weather above 80 degrees so you have to start very early in the spring.