Eg. for Pizza.
Boil some big tomatoes for 5 minutes. Take them out and let them cool. When they are a bit cooler, you can peel them easily. Fry some cut onions and/or garlic and add the peeled and smashed tomatoes. Add also half teaspoon of sugar (so it doesn't get sour), salt as needed (probably quite much), spices (oregano, basilica and so on) and extra olive oil.
Let it cook for about 2 hours until it becomes sauce'ish.