Use the jar lifter to remove the warm jars from the canner, drain, and line up on a towel on the kitchen counter.
Add 1/2-teaspoon citric acid or 2 tablespoons lemon juice to each quart sized jar (1/4-teaspoon citric acid or 1 tablespoon lemon juice for pints).
Use the canning funnel and ladle to fill the jars with the tomato puree. Leave a 1/2-inch headspace.
Run the bubble popper through the jars to release any air. Wipe the rims of the jars clean with a damp paper towel.
Use the lid lifter to remove the lids from the warm water, add to the tops of the jars, apply bands, and screw on until fingertip tight.
Use the jar lifter to place the jars back into the canner leaving space in between them.
Once all the jars are in canner, adjust the water level so it is at least one inch above the jar tops. If you need to add water, use the hot water from the small pot your lids were in. Pour the water around the jars and not directly on them.
Cover the canner and bring to boil over high heat. Once water boils vigorously, continue boiling quart size jars for 40 minutes (35 minutes for pint jars) at altitudes of less than 1,000 feet (Adjust processing time for your altitude if necessary. See note below.).
When processing time is complete, turn off the heat, remove the cover, and let the canner settle for about 5 minutes.
Spread a kitchen towel on the counter. Use the jar lifter to lift the jars from canner and place on the towel.
Don't tighten bands or check the seals yet. Let the jars sit undisturbed for 12 to 24-hours to cool. You will hear the satisfactory "ping" of the jar lids sealing as they cool.
After 12 to 24-hours, check to be sure jar lids have sealed by pushing on the center of the lid. The lid should not pop up. If the lid flexes up and down, it did not seal. Refrigerate the unsealed jar and use in a week.
Remove the ring bands, wash, label, date the jars, and store the canned tomato puree in a cool, dark location for 12 to 18 months. Once the jar is open, refrigerate and use up within a week. Yields about 7 quarts of tomato puree.