– Fully ripe tomatoes
– Small jar – Fine mesh strainer – Paper towel – Small plate – Small envelope
Squeeze or scoop the seeds (with accompanying gooey stuff) out of fully ripe tomatoes into a clean, small jar. There's no need to separate the seeds, but do try to remove as much of the pulp as possible.
Cover the seeds completely with at least an inch of dechlorinated (or filtered) water and loosely place a lid over the jar. Do not seal the jar tight, as you want to leave some airflow for the fermentation gases to escape.
Within a day or two (more or less, depending on your ambient room temperature; warmer temps move the fermentation along quickly), a frothy white film will start to form on the surface of the water. It will smell slightly sour, like yogurt.
Once the white film has covered the whole surface (in another day or so), it's time to wash away all that scum.