To can Roma tomatoes, lightly wash the tomatoes, and remove the stems. Make two crossing cuts at the bottom, and put them in boiling water for one or two minutes. Place the tomatoes in an ice bath. After a few minutes, peel off the skin. Prepare the Mason jars beforehand by running them through a dishwater or by putting them in a pot with water and letting them boil. Sterilize the lids by boiling them between three and five minutes.
Fill warm Mason jars up to 1/2 inch of the rim, and compact the tomatoes with a spoon. Add 1 teaspoon of lemon juice for pint-sized jars and 2 tablespoons for liter- or quart-sized jars.
Make sure there are no air bubbles in the jars and that they are filled up to 1/2 inch of the rim after compacting the tomatoes. Carefully wipe off the rim of the jar with a paper towel or clean cloth. Add lids, and secure with rings just until the rings are finger tight.
Use a canner or a big stock pot to process the jars. Put a clean rack inside the pot to ensure the jars do not sit on the bottom of the pot. Fill the pot with water, and place the jars inside, making sure the water level is a minimum of 2 to 3 inches above the lids. Cover the pot with a lid. Bring the water to the boil, and let the jars boil in the water for 85 minutes.
Remove the jars with a jar lifter, and put them in a draft-free place for 12 hours. If the lids do not seal, refrigerate the jars, and consume the tomatoes within a few days. If the lids are flat, store them for later use.