Choose a fully ripe, healthy tomato
Cut the tomato open and scoop out the seeds with the surrounding gel
Place the seeds and gel into a clean jar or cup
Add a small amount of water if needed to cover the mixture
Let the mixture ferment at room temperature for 2 to 4 days
Stir once a day
Stop fermenting when a layer of mold forms on top and the seeds sink
Add more water and pour off the floating pulp and bad seeds
Repeat rinsing until only clean seeds remain
Spread the clean seeds on a paper towel, coffee filter, or plate
Dry the seeds completely for 1 to 2 weeks
Store the dried seeds in a labeled paper envelope or airtight container
Keep the seeds in a cool, dark, dry place