Generally what we do in our family is cheat a little. You need to get canned tomato paste, because that's what gives it consistency... then you chop the tomatoes up really small, and add all the spices and stuff you want(onions, garlic, basil, oregano, a pinch of sugar, and, if you want, mushrooms, and/or meat) and simmer it in a big pot until it tastes good. I know it's not precise, but that's how we've always done it.....