Scald & peel ripe tomatoes. Place in jars. Add 1 teaspoon salt to 1 quart, tighten by hand. Place on cookie sheet in oven 4 to 5 inches from bottom of oven. Turn oven to 250 degrees for 1 hour. Turn off oven & let cool in oven.
Once in while I've had one not seal. I just put a new flat on the jar, screw down ring & put it back into the oven & do it again.