Slice the tomato in half and scoop out the insides--the seeds and their surrounding "gel"--and place in a clean container.
Add several inches of water to the container. This will help the gelatinous seeds to ferment.
Leave the container in a warm spot for about three days.
When seeds are fully fermented, they'll have a moldy, scummy look. At this point, discard the moldy film and any seeds that float to the top and rinse the remaining seeds in a sieve or strainer in the sink. Remove any traces of film until only the seeds remain.
Dry the seeds in a single layer on a plate lined with waxed paper.Turn the seeds several times in the next few days to thoroughly dry them. Drying may take from one week to up to two weeks, depending on the climate.
Seeds are completely dry when they do not stick to the waxed paper or each other.