Fresh ripe tomatoes
Core the tomatoes and gently score a small X, cutting just through the skin, on the bottom of each tomato. In a large pot, bring some water to boil. When the water is boiling, put the tomatoes in until the peel starts to peel up at the edges. Put the tomatoes in an ice bath to stop the cooking.
In your canner, start water boiling to sanitize the lids and jars. I sometimes use a tea kettle to boil water to sanitize the jars. In order to can, you must plan and multi-task or it will take you FOREVER.
While water is boiling and pieces are sanitizing, you can peel and dice the tomatoes. Put the cutting board inside a baking sheet with walls so you don't lose any of the juice while dicing tomatoes. You need the juice for your cans. This also makes clean up much easier. Discard the peels. I used a hand chopper to help with dicing the tomatoes. But you have to be careful with a hand chopper. If you chop too many times, you'll end up with mashed tomatoes or tomato sauce instead of diced tomatoes. Put your diced tomatoes with their juice in a large pot and bring to a low simmer. The directions I had said to boil for 30 minutes but I didn't cook it for quite so long. You can tell when to stop the boiling. The tomatoes start to foam and get very mushy. They will cook more during canning so you don't want to overcook them now.
While the tomatoes are simmering, juice the lemons. All of the instructions I have said to use citric acid or bottled lemon juice. But, citric acid that you just buy from the store, is usually made with corn ingredients. And I don't trust bottled lemon juice anymore either unless I know the source for sure. So I juiced the lemons myself. Juice several. It is better to have too much than not enough. You can always freeze the leftover lemon juice in ice cube trays to use later for marinades and juices.
Now, ladle the diced tomatoes into the sanitized jars leaving plenty of head space at the top of the jar. Add 1 Tbsp. of the fresh squeezed lemon juice to every jar of dice tomatoes. Use a small rubber spatula or chopstick to clear out the air bubbles and mix in the lemon juice. Tighten the lids and follow the directions for water bath canning in your canner. They should be in the canner for around 35 minutes.
After they are out of the canner, I usually tighten the lids every couple of hours for the rest of the day.
And here is a picture of my lovely canned diced tomatoes: