Chop the top of the tomatoes. Quarter them. Blend them in a food processor. They go pink!
Fry the finely chopped onion and crushed garlic until it smells absolutely gorgeous.
Add the tomato gunk.
Stir and increase the heat.
When it is boiling (when you can see a bunch of bubbles), bring the temperature down so you only see a few bubbles every second or so.
(This is sometimes called "simmering"!)
Let it simmer for an hour, check on it and stir it but you can leave it to it.
You're trying to evaporate the moisture so it is the thickness you want. So. When it is the thickness you want, take it off the heat.
You can now use it for pizza and pasta or whatever.
Be sure and put some in some small jars that you have recycled for this very purpose. Put the jars in the freezer.