Have ready 4 very clean pint jars with new lids and screw-top bands that have been simmered in hot water; keep submerged in hot water until ready for use.
Bring a large pot of water to a boil. Add tomatoes, cook 30 seconds, then remove. With a paring knife, slit the skin of the tomatoes; peel off skin over a colander placed over a bowl. Quarter tomatoes over colander, and with your thumb, press out seeds. Set aside seeds, skins, and juice.
Place 1/2 teaspoon salt and 1/4 teaspoon citric acid or 1 teaspoon lemon juice in each hot empty jar. Pack in tomatoes (a pint jar should hold 1 1/2 large tomatoes or 4 to 5 small ones). Press down to release the juices; if the tomatoes aren't juicy, add enough boiling water to fill the jar. In either case, leave 3/4-inch space at top. Free any air bubbles by running a butter knife around inside of jar.
Wipe rims, then screw on lids with bands until snug but not tight. Place jars in a water-bath canner or on a rack in a large pot, and cover with 2 to 3 inches water. Bring to a boil, then lower heat to maintain a rolling (not violent) boil. Boil tomatoes 40 minutes at sea level, adding 2 minutes for every 1,000 feet of altitude. (Jars must remain covered with water throughout processing time.)
Turn off heat. Wait 5 minutes, then remove jars using tongs, and let cool; you may hear the lids ping, a sign that vacuum sealing has occurred. After 8 hours, check each lid to make sure it is concave (another sign that it's vacuum sealed), then remove screw-top band and try to remove lid with your fingertips. If you can't, the seal is tight. Label and date jars, then store in a cool, dark, dry place up to 1 year (after opening, tomatoes can be refrigerated up to 1 week). Before using, check to see if lid is still concave; if it isn't, discard.
Press seeds and skin through colander to extract as much juice as possible (5 tomatoes should yield about 1 cup juice). You can process this juice the same way as the tomatoes or use it immediately to make a Bloody Mary.