Spread a kitchen towel on the counter. Use your jar lifter to remove a jar from canner, drain, and place on the towel. Keep the remaining jars in the canner so they stay hot.
Add 1/4 teaspoon of citric acid or 1 tablespoon of bottled lemon juice to each pint sized jar. (Add 1/2-teaspoon citric acid or 2 tablespoons lemon juice to each quart jar.)
Add up to 1/2 teaspoon of salt for pint jars (1 teaspoon of salt per quart), if desired for flavor.
Use the canning funnel and ladle, and fill the warm jars with hot tomatoes leaving 1/2-inch headspace.
Run the bubble popper through the jar to release air. Wipe the rim clean with a damp paper towel.
Center a lid on the jar, and screw on the band until it is fingertip tight. Use the jar lifter to place the jar back into the canner, and repeat with the remaining jars.
Once jars are all in canner, adjust the water level so it is at two inches above the jar tops.
Cover the canner and bring to boil over high heat. Once water boils vigorously, process pint sized jars for 35 minutes (quart sized jars for 45 minutes) at altitudes of less than 1,000 ft. Adjust processing time for your altitude if necessary. (See notes)
When processing time is complete, turn off heat, remove the cover by tilting lid away from you so that steam does not burn your face, and allow the canner to cool down and settle for 5 minutes.
Spread a kitchen towel on the counter. Use a jar lifter to remove the jars one at a time from the canner. Keep the jars upright, and place them on the towel. Do not tighten ring bands or test the seals yet. Let the jars sit undisturbed for 12 to 24-hours to cool.
After the jars have cooled for at least 12 hours, check to be sure 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 jar and use within a week.
Remove the ring bands, wash, label, date the jars, and store the canned tomatoes in a cool, dark place between 50 to 70 degrees F for 12 to 18 months. Once the jar is open, refrigerate and use up within a week. Yields about 9 pint jars of crushed tomatoes.