There are multiple ways to do this:

Dishwasher Method - Some dishwashers have a sanitize function on the dishwasher which works well for preparing the mason jars. The downfall, however, is this can be a time consuming method.

Oven Method - Wash the jars with soap, rinse, but do not dry. Place upside down on a baking sheet and put in the oven for 20 minutes at 275 degrees Fahrenheit (130 degrees Celsius). Carefully remove the hot jars for canning.

Boiling Method - Wash the jars with soap and rinse. Place jars in a large pot of boiling water (like the water bath canner) and let them sanitize for 20 minutes. Remove jars and let air dry before use.

For the lids - place in a small sauce pan of hot water until it is time to add them to the jars.

Preheat oven to 350 degrees.

First wash the tomatoes. Then remove the stem and core, and quarter the tomatoes. Leave the tomato skins intact. Place tomatoes into an aluminum cake pan or 9x13 baking dish. Peel and chop the onion, garlic, and other herbs. Add to the baking dish atop the tomatoes. Add a generous amount of salt and pepper.

Bake tomatoes for 1 hour. Remove from the oven and allow the tomatoes to cool slightly.

Using an immersion blender, purée the tomatoes until they are a smooth consistency.

Add hot water to a little over ½ of the water bath canner and bring to a boil.

Using a canning funnel and a ladle, ladle sauce into the sterilized jars, leaving 1 inch of headspace. Remove air bubbles from the hot sauce and wipe the rim of the jar with a clean damp cloth or papertowel. Using the magnet lid lifter tool, grab the jar lid from the small sauce pan, and carefully place it on the jar. Loosely screw the rings onto the lids to ensure the lids stay in place.

Place the jars on the inner wire rack of the water bath canner and lower it into the boiling water. Add the lid to the canner. Process for 35 minutes in boiling water.

After the 35 minutes is up, turn off the heat and carefully remove the canner lid. Grab the metal insert by the handles (with oven mitts) and raise the jars up, carefully hooking the handles onto the edge of the water bath canner. Let the jars sit for 5 minutes.

Lay out a hand towel or a doubled up flour sack towel on a counter surface. This will act as a barrier between the hot jars and cool counter, preventing the counter and keeping the jars from bursting with the drastic temperature change. Using the jar lifter tool, carefully remove the jars from the wire insert and place them on the towel. Allow jars to sit and cool, undisturbed, for 12-24 hours.

Test jars for airtight seals. The lids will not “pop” when pushed down on and will not pop off when pulled up on.