Prep your canning equipment by washing your jars, lids, and screw bands.
Sterilize the jars by submerging them in hot water.
Add the lids to a separate pot of warm water and keep them warm.
Fill your water-bath canner half full of water and heat it to simmering. Screw bands do not need to be kept warm.
Add 2 tablespoons of bottled lemon juice to each jar. Do not use fresh lemon juice.
Using a funnel, transfer the spaghetti sauce into the jars. Leave ½ inch headspace at the top. Release any air bubble with a spatula.
Place a lid on each jar and hand tighten a screw band on. Do not over-tighten. Repeat until you've canned all of your tomato sauce.
Place your jar rack in the water bath canner, suspending it with the handles on the edge. Then put your jars in the rack.
Unhook the jar rack and carefully lower it into the hot water. Add more water if necessary to cover the jars with at least 1 inch of hot water. Heat the water to a full rolling boil for 40 minutes.
Remove your jars with a jar lifter and place them on a clean, dry kitchen towel away from drafts and allow them to cool completely for 12-24 hours. Test to make sure each jar has sealed and store in a cool, dark place.