To eliminate any possibility of immature seeds, simply pick your tomatoes when they have a little color to them. (Though I do recommend waiting for your tomatoes to ripen on the vine or ripening them indoors first, just so you don’t “waste” a perfectly good tomato.)
Cover the seeds completely with at least an inch of dechlorinated (or filtered) water and loosely place a lid over the jar while the whole thing is brewing.
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.
The cloudy white film isn’t mold; it’s harmless kahm yeast, a common indicator of lacto-fermentation in action. While it doesn’t look like anything significant is happening, it’s also a sign of your potential new seedlings and next year’s salsa (which you can also ferment).