First, when we get tomatoes from the garden or our CSA, I typically cut them into chunks, and toss them on a baking sheet with four or five cloves of garlic and enough olive oil to coat them. After roasting them for a few hours at about 250 degrees, I put them in pint jars (don't fill them to the top, or they'll break) and pop them in the freezer.
When I'm ready to make sauce, I thaw out a few jars. Then, I warm a little olive oil in a big pot. I add about half of a diced onion, and let it cook until the bits are just a little soft. Next, I add the tomatoes, and often a few more cloves of garlic. I also stir in a handful of diced herbs. This varies based on what we have. Often, it's oregano from the garden or a basil cube from the freezer. Lastly, a little tomato paste can be nice to make the sauce thicker. I usually don't have any--so it's fine without--but put a big spoonful in if you have some on hand.
I let the sauce simmer together until it's bubbly and warm. It largely depends on how much time I have. Longer is better, but 15 or 20 minutes is plenty good for a busy weeknight. (That's why I pre-roast the tomatoes--so that I don't have to cook them as long later.) Depending on how juicy the tomatoes are, I sometimes add about a half of a cup of water as the sauce cooks. If it looks like it's sticking to the bottom of the pot, add water; if not, just stir it gently every few minutes.
The sauce is good when you leave the tomatoes in chunks, but sometimes I also use my immersion blender to break up the tomatoes. Again, it largely depends on what mood I'm in, how much time I have, and whether the Kid is otherwise occupied or not.