Fill your jar with tomatoes. For a quart jar, you'll use about 3 cups of tomatoes, to end up 1 ½ inches below the rim. For any other size, just fill the container to 2 inches below the rim.
Top with a fermentation weight.
Make a brine by mixing a pint of water with 1 tablespoon of salt and shake or stir till it's dissolved.
Pour the brine over the tomatoes. Be sure to cover all the tomatoes with brine. They'll want to float, and that's why we need to put a weight on top. But they all need top end up under the brine surface when you're done.
Seal your jar with a lid of your choice. If you use a fermentation lid, there is a handy little date marker on it that you can set. Mark any others with today's date.
Place in a darkish spot to ferment. Pick a darkish corner in your kitchen, where it's out of the way for the next 1-3 weeks, but where you'll be reminded to keep an eye on it. It should be out of direct sunlight for sure. The best temperature for fermenting is between 72 and 80℉. A little less or more is ok, but definitely keep it above 65, or your fermentation might not kick in. Too hot and you might end up with mush due to over-fermentation.
Observe for 1-3 weeks. If you use cloth to cover your jar, be sure to keep an eye on the top of the ferment. You might have kahm yeast forming, which will need to be skimmed off regularly. It's harmless, but it is unsightly and it can give your ferment a bit of an off-flavor if it's left on too long.If you're using a sealing lid, burp it once a day to let the pressure escape from inside, but don't take the lid off if you don't have to. Just quickly untwist and reseal.You don't need to babysit it, if you're using a fermentation lid or a Fido jar. Smell and taste your tomatoes after one week. Before you decide to put them into cold storage, you'll want to make sure that they have a distinctly acidic taste. They'll probably be a bit fizzy too, like champagney, if that's a word. If not, let them go a while longer. Once they reach a strong tangy flavor and have that distinct fermentation aroma (you'll know what I mean when you smell it), you can place it into cold storage. In any case, to increase the lactic acid and probiotic benefits, let them ferment for about 3 weeks.
Place into cold storage. Cold storage can be your root cellar, a cool spot in your home, or the refrigerator. This will slow the activity way down and the cooler it is, the longer it will keep. It should keep for a year with no problem, or even longer. Do try to keep it under about 55 degrees for long term storage.