Inspect mason jars for cracks or chips, then wash them and keep them hot until ready to use.
Wash new lids and rings with hot, soapy water.
Fill your water bath canner with enough water to cover jars by 1-2 inches and begin heating.
Remove tomato skins by blanching in boiling water, then core and crush the tomatoes. Place crushed tomatoes in a large pot, bring to a boil, and simmer until thickened, stirring often.
Add bottled lemon juice or citric acid to each hot jar to ensure safe acidity.
Fill hot jars with hot tomato sauce, leaving ½ inch headspace; remove air bubbles and wipe rims clean.
Apply lids and rings fingertip-tight, then place jars in the canner and process (pints: 35 min; quarts: 40 min, plus altitude adjustments as needed).
Remove jars, cool undisturbed for 12-24 hours, check seals, then label and store in a cool, dark place.
