Soak the seeds and pulp in water for about two days or until they start to ferment, but not long enough to sprout. Carrots, radishes, and other biennials will not produce seed until the second year of growth. These can be left in the garden over winter under a heavy mulch, or they can be dug and stored in damp sand in the root cellar for spring replanting. If they will be in the way of future tilling or succession planting, consider a separate area for seed production. When the seed head is almost ripe, tie a paper bag over it to collect those seeds that will otherwise fall to the ground.
Onions are also biannual, but producing bulbs or sets requires yet another year.
When the tops fall over pull them and store them in net bags for planting the following year.