One of the easiest seed to save is cress /karse (Lepidium sativum). You can grow a lot of seed in a small space, it dries well on the plants and is easy to process, rubbing the seed pods between your fingers in a bowl and then blowing the chaff away outside. I sprout the seed inside during winter and also sow it outside in spring to harvest the young plants for salads and lunch. leaving a few to grow big for seed.