Cut them open and squeeze the pulp and seeds into a deli container. What works well for me is to put the container in the bottom of a bucket, and hold the tomato down inside the bucket as I work the seeds and pulp with my hands. Don't fill the container more than about halfway or the fermentation won't have room to go to completion. Place the uncovered container out of the sun in a place where critters won't tip it over, and where the ripe odor and fruit flies associated with fermentation won't bother anyone. Do not stir.
Some people remove the fungus layer and strain the pulp and seeds through a sieve.
This method has never worked well for me. I remove the fungus layer, fill the container with a gentle spritz from the hose, swirl, and pour off the upper layer. Repeat this process until the water stays clear and the good seeds have settled to the bottom. Knock out the seeds onto a labeled paper plate and put them some place out of the sun until thoroughly dry. This usually takes at least a week.