My kids eat a lot of ketchup. I think most kids do, so nothing super weird there, but commercial ketchup contains high fructose corn syrup which is something we try to avoid. So I make my own. Not to mention that I think it tastes better in most applications. It is different, far different, than the stuff you get in the squeeze bottle, but it is healthier and yummier.
I will probably put up 14 pints of ketchup this year. To do that I need 48 lbs of tomatoes, some sort of paste tomato. A paste tomato is something that has a lower water ratio than a slicing tomato. Rome tomatoes are the most common, but cherry and grape tomatoes also work. I personally use an heirloom variety called "Amish Paste" They are perfect and have a fantastic tomato flavor that really shines through in all my tomato stuff. I also have a yellow pear tomato plant in my backyard which is producing TONS of tomatoes, far more than we can eat, so I threw some of those in my ketchup also. The yellow tomatoes have a very intense sweet tomato flavor, and I think they actually added a nice flavor to my ketchup. If I continue to get this huge crop I might make a special yellow batch of ketchup to see how that turns out.
For many tomato applications you need to first peel the tomatoes. Tomatoes are very easy to peel. Drop them in boiling water for a minute, then straight ice water. The peels slip right off. However for sauce and ketchup you don't need to do that. Both of those applications go through a food mill, so you can just chop them in quarters and cook them until soft. While you are cooking the tomatoes you also need to add some onions. If you have 24 lbs of tomatoes you need to add three large or four medium onions and about ten cloves of garlic (I love garlicky ketchup) Combine the quartered tomatoes and onions and garlic and cook until soft.
While this is cooking (over the lowest heat possible) take 3 cups of apple cider vinegar and put it in a stainless steel pan. Make a spice bag with 3 tablespoons of celery seed, 2 cinnamon sticks broken up, 2 teaspoons whole allspice, 2 teaspoons whole cloves, and one teaspoon dried red pepper. Bring the vinegar and spice mixture to a boil and as soon as it hits a boil, turn off heat and let stand for one hour.
*a note about vinegar. Use organic or raw vinegar when cooking. Other vinegars can be full of weird stuff like formaldehyde.
Combine the vinegar and tomatoes and let stand for 3 hours at room temperature. Add three to four tablespoons of canning salt at this point (kosher salt works also, but table salt does not due to the iodine in it) This slightly pickles the tomatoes and makes the flavor deeper. At this point you need to run your mixture through a food mill. I think that some of the best 50 dollars you can spend is on a good food mill and my favorite for large batch cooking is this food mill
At this point I put the ketchup mixture in my crock pot and put it on low overnight. This helps to keep the ketchup from burning, which is possible on the stove top. I don't use my lid, but rather cover the opening with cheese cloth to allow the moisture to escape. When I wake up in the morning the kitchen smells amazing, and I have perfectly thick ketchup.
I pack mine in pint size jars, with clean sterilized lids and process in a boiling water canner for 15 minutes.