Heat oven to 250° and place clean canning jars into oven on a baking sheet.
Remove lemon juice from fridge and allow to warm to room temperature (if it's cold when starting out). This prevents the jars from breaking when you add the cold liquid to the hot jar.
Skin and de-seed tomatoes. This is easiest if they were frozen first!
Remove blossom end with a knife.
Place tomatoes in a large stock pot and bring to a boil.
Using an immersion blender(if available) blend tomatoes until smooth. If no immersion blender is available, use a regular blender or a food processor to blend tomatoes. Transfer back to the pot and bring to a low boil.
Remove jars from oven and add lemon juice to bottom of each jar. 1 tbsp per pint, 2 tbsp per quart.