Step 1: Prepare your canning equipment. Wash jars, lids, and rings thoroughly. Sterilize jars by submerging them in boiling water for 10 minutes or heating in an oven at 225°F (107°C) for 20 minutes. Keep jars hot until filling. Heat lids in simmering (not boiling) water as per manufacturer's instructions.
Step 2: Wash tomatoes well. Score a small 'X' on the bottom of each tomato. Bring a large pot of water to a rolling boil. Carefully blanch tomatoes for 30-60 seconds until skins crack. Immediately transfer to an ice bath to cool.
Step 3: Once cool enough to handle, peel the skins off the tomatoes. Core them (remove the stem end) and chop them into desired pieces. For a smoother product, you can remove seeds if preferred.
Step 4: Place the chopped tomatoes in a large, heavy-bottomed pot. Bring to a boil over medium-high heat, then reduce heat and simmer gently for 15-20 minutes for crushed tomatoes, or longer (up to 45-60 minutes) for a thicker sauce. Add canning salt to taste during simmering.
Step 5: Add the acidifier to each hot, sterile jar before filling. Use 1/4 teaspoon citric acid or 1 tablespoon bottled lemon juice for pint jars. Use 1/2 teaspoon citric acid or 2 tablespoons bottled lemon juice for quart jars.
Step 6: Carefully ladle the hot tomato mixture into the prepared jars, leaving 1/2 inch of headspace. Remove air bubbles by running a non-metallic utensil around the inside of the jar. Wipe jar rims clean with a damp cloth. Center hot lids on jars and apply rings, tightening to fingertip-tight.
Step 7: Place filled jars into a boiling water canner, ensuring they are covered by at least 1 inch of water. Bring water to a rolling boil. Process pint jars for 35 minutes and quart jars for 45 minutes. Adjust processing time for altitude if necessary (add 5 minutes for every 1000 feet above sea level).
Step 8: After processing, turn off heat, remove canner lid, and let jars sit in hot water for 5 minutes. Carefully remove jars with a jar lifter and place them on a wire rack or towel-lined surface to cool undisturbed for 12-24 hours. Check seals; lids should be concave and not flex when pressed. Unsealed jars must be refrigerated and used within a week or reprocessed. Store sealed jars in a cool, dark place.