Choose fully ripe, healthy tomatoes from open-pollinated or heirloom varieties
Cut the tomato and scoop out the seeds with the surrounding gel
Place the seeds and gel in a clean jar or container
Add a small amount of water to cover the mixture
Let it ferment at room temperature for 2 to 4 days
Stir once a day and stop when a layer of mold appears on top
Add more water, then pour off the floating pulp and bad seeds
Rinse the good seeds thoroughly in a fine sieve
Spread the seeds in a single layer on a paper plate, coffee filter, or screen
Dry the seeds completely in a cool, dry, shaded place
Store the dried seeds in a labeled paper envelope or airtight container
Keep the container in a cool, dark, dry location
Use within 4 to 6 years for best germination