STEP ONE: Prepare your canning equipment and ingredients. To make tomato sauce, you'll need: tomatoes, bottled lemon juice and salt. If you are using heirloom tomatoes, be sure to include more red than yellow tomatoes. Red tomatoes have more acid.
STEP TWO: Prepare your tomatoes: Score them, and then blanch in boiling water for about a minute. Put blanched tomatoes into bowl of ice water to allow them to cool.
STEP THREE: Peel your tomatoes and quarter or half them depending on their size. Crush the tomatoes with your hands. Boil crushed tomatoes and their juices for 10 minutes. Stir as needed. Run your tomatoes through a food mill or a Chinois to strain the seeds.
STEP FOUR: Return sauce to a boil and reduce by half. Salt to taste.
STEP FIVE: Prepare your clean jars for canning by submerging them in pot of boiling water (with a rack or towel on the bottom, so the jars don't touch metal of pot directly). This will heat your jars up so they don't break later.
STEP SIX: Try to use tongs to take the jars in and out of the pot.
STEP SEVEN: Remove your jars from the hot water and quickly add 1 Tablespoon of bottled lemon juice to EACH pint jar. This will help keep the acid content high enough to avoid problems.
STEP EIGHT: Ladle your tomato sauce into your jar leaving room at the top for head space so the sauce can expand during the boiling water processing.
STEP NINE: Wipe edges of jars. Place the flat part of the lid on the jar, then twist the screw top just until tight. Don't over twist.
STEP TEN: Put jars of tomato sauce into boiling water, process for 35 minutes for pint jars (at sea level) and 40 minutes for quart jars (at sea level).
STEP ELEVEN: When water bath is complete, turn heat off and let jars sit for 5 minutes. Remove the jars from the water and let cool on a rag or a cooling rack.
STEP TWELVE: Tomato sauce! Congratulations! You'll hear a completely pleasing popping sound as the jars cool and the pressure inside pulls the lids in.