Stem them, and trim off and discard bruises. Don't peel or seed or core.
Take 6 of the tomatoes, cut into quarters and add to a large pot over high heat.
Crush them with a potato masher or other similar tool.
Stirring the pot frequently and keeping the heat high, continue to quarter the remaining tomatoes, adding them as you do and crushing them, so that they heat rapidly, too, and the mixture keeps on boiling, uncovered.
When all the tomato is in, let simmer strongly for another 5 minutes.
Pass tomato mixture through a sieve or food mill to remove skin and seeds.
Put juice back in pot, bring back to a boil, then lower to a simmer to keep quite hot.
MANDATORY. Acidify jars. To each half-litre/ pint jar add either 1 tablespoon bottled lemon juice OR ¼ teaspoon citric acid. To each litre/quart jar add either 2 tablespoons bottled lemon juice OR ½ teaspoon citric acid.
OPTIONAL: ½ teaspoon salt per half-litre/ pint; 1 teaspoon per litre/quart jar.
Ladle hot juice into jars.
Leave 2 cm (½ inch) headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process half-litres (pints) for 35 minutes; litre (quart) jars for 40 minutes. Increase time as needed for your altitude.