Canning tomatoes is a very simple process! Like really, really simple.
Add water to your canner and lightly warm it on the stovetop.
Peel your tomatoes (this is super easy if you froze them!), dice them, and pack them into canning jars that have been warmed in the oven.
After putting in the tomatoes, add one T. of lemon juice to each jar, fill with hot water to leave 1 inch of headspace, wipe the rim, put on a lid and ring, and place in the steam canner.
Follow the directions for your canner for processing the diced tomatoes. My canner has you add 6 cups of water to the pot, add your jars, and process pints for 40 minutes after is fully heated (steam should come out of the hole in an 8 inch stream when it's heated through).
After you are done processing, remove the jars and allow to cool on some stacked towels. Lids should pop sealed and those jars once cool can be stored with rings removed. Any jars that do not seal should be put in the fridge and eaten in the next few days!
** Note: I can diced tomatoes in a steam canner. Not enough research has been done on the steam canner for the USDA to say that steam canning is 100% safe; however, we have chosen to use this method as it requires less water and we have never had any problems with the canning method. If you are uncomfortable with steam canning or do not have a steam canner, you can easily find the method for water bath canning.