I didn’t ever seem to grow enough tomatoes to can tomato sauce, so I have had to buy some. I like buying the larger # 10 cans they are a lot cheaper this way. The only problem with this is when you open them you need to be able to use all of it with in a few days. So I can mine into smaller quart and pint jars. It is very easy and doesn’t take to long.
Pour however many cans you want to do in to a large pan. ( there is about 12 1/4 cups in a can which is a little more then 3 quarts).
Warm over a medium heat until hot be careful it will splatter when it bubbles up.
I don’t know if you really need to do this but I go ahead and add two tablespoons per quart ( 1 tablespoon per pint) of lemon juice ( you could also use vinegar) to make sure it had enough acid, since I was going to use the water bath. I put the lemon juice in the bottom of the sterilized jars and then poured the tomato sauce in. Put the hot lids and rings on then water bath 40 minutes for quarts and 35 for pints (be sure to adjust for your altitude I have to do my quarts for 55 minutes and my pints for 50 minutes) You can also pressure can them for 20 minutes.
Now you have doable sized jars of tomato sauce, and they are a lot easier to handle and store then the big cans.