Place your canning jars in an inch or two of hot water to keep them warm. Keeping the jars warm reduces the temperature change when you're packing your jars, which is important to reduce the chance of the jars breaking later. You can keep them in the sink in a few inches of hot water, or run a hot rinse cycle on your dishwasher and keep it closed until you're ready to fill the jars.
Wash the tomatoes and dip them in boiling water for 30-60 seconds or until you see the skins split. When you remove the tomatoes, drop them immediately into a bowl of cold water to stop the cooking. The skins should now just slide off in your hands.
As you skin the tomatoes slice them in halves or quarters as you prefer. If they're small enough to fit into the jars you can also keep the tomatoes whole.
Place them directly into your jars, pressing down on the tomatoes in the jar until spaces between them fill with juice.
When the jars start to fill be sure to leave a 1/2-inch of head space. Repeat steps until all tomatoes are skinned and chopped and packed in jars.
Remove air bubbles by giving the jars a small, gentle stir with a long utensil.
If you are canning tomatoes in a water bath canner, add 1 tablespoon per pint or 2 tablespoons per quart of bottled lemon juice or citric acid to the jars. Add 1 teaspoon of salt per quart to the jars, if desired.
Be sure and wipe the rims of your jars clean before placing your lids on. If there are bits of food it may interfere with the seal. Place lids on the jars and process according to Water bath or Pressure Canning instructions.
For Water bath canning, heat a large pot to a gentle boil, then add the warm, sealed jars to the water til they're submerged. Keep the water at a gentle boil for 85 minutes, increasing time for higher altitudes.
For altitude adjustments, if you're between 1000 - 3000 ft increase boiling time by 5 minutes. For 3000-6000 ft, increase time by 10 minutes. 6000-8000 ft, increase time by 15 minutes. 8000-10000 ft, increase time by 20 minutes.
