When I can Diced Tomatoes etc I save the peels, cores, tops, seeds etc then Juice them in my Juicer or run it through a Food Mill then I can the juice. I use this juice for soups or to thin out sauces etc.
Wash tomatoes and prepare like THIS. Cut tomatoes into quarters and simmer till they become soft, make sure to stir to keep the tomatoes from sticking. Then Juice tomatoes in juicer, food proccer, or food mill. Strain juice (I put cheese clothe over my strainer) to remover any peels, or seeds that may still be intact. Heat the juice to just below a boil for 5 minutes.
In Hot Jars add 1 Tablespoon bottled lemon juice per pint and 2 Tablespoons per quart . Then fill with juice laving 1/4 headspace. place on lids and process pints for 35 minutes and 40 minutes for quarts In a water bath.