Start with tomatoes that are fresh, ripe and firm. You don't want to use overly ripe tomatoes, because their acid levels may be too low, and unsafe for canning.

Wash, core and remove any bruised or green spots from the tomatoes. Take a third of the tomatoes and cut into quarters.

Quickly add the quartered tomatoes to a pot over medium-high heat. Using a clean potato masher, gently crush the tomatoes until they start to break down. Once the tomatoes are cut, they start to release pectinase enzymes, which will cause the tomato juice to separate. Heating kills the pectinase enzymes, so you want to put heat on the tomatoes as soon as they're cut.

While they are cooking, quarter another third of the tomatoes, and add those to the pot, crushing and stirring as you go.

Quarter and add the remaining third of the tomatoes and continue crushing until all the tomatoes are broken down.

Reduce heat to medium and allow them to simmer for 5 minutes.

Remove from heat. Using a sieve or food mill, separate the juice from the seeds and peels.

Return the juice to the pot and bring to a boil. Remove immediately and ladle into sterilized jars, using a sterilized funnel, making sure to leave 1/2" of headspace. To each jar, add 1 tbsp of bottled lemon juice or 1/2 tsp of citric acid. You can also add 1 tsp of salt to each jar for taste, but that is optional.

Wipe the rims of the cans with a clean paper towel and top with a sterilized lid and rim and tighten until finger-tight. Fill your water bath canner with water and bring to a boil.

Process in water bath canner for 35 minutes.

Carefully remove, and allow to set undisturbed until the jars have properly sealed.