I didn't realize that I have never really described how to make tomato sauce. Here's are some basic instructions, although no pictures (sorry). Keep in mind, this is a very plain tomato sauce - the idea is that you add whatever else you want (i.e. onions, mushrooms, etc.) when you actually use the sauce.
10 kg tomatoes
A few handfuls of basil
Lemon juice optional
1. Chop up the tomatoes and toss them in a big pot (or two if necessary). Put this on the stove. My husband's family recommends squeezing them to get some of the excess liquid out (to have a saucier sauce). I use roma tomatoes only, so far, which don't really have excess liquid at this stage, so I never do this.
2. Add some salt (a small handful or so), turn the heat to medium, and cook for 30-40 minutes. Do not cover - you want some of the liquid to boil off.
3. Straing the sauce to lose some of the water. I use a T-shirt in a colander and strain in batches. Don't use a T-shirt that you plan on wearing ever again. I used to do a double-layer, now I just use one layer of T-shirt. I also try to pour off any really watery stuff that doesn't strain out.
4. Pass the tomatoes through a tomato press or sauce machine, such as this one, to remove the skin and seeds. At this ponit, if your sauce is really watery still, you can cook it down some more if you want. I never do (maybe I should, though).
5. Chop up the basil and stir it in. You can do this earlier, but I find that I lose a lot of it in the the sauce machine.
6. Process in a water-bath canner. Add lemon juice if you want - the USDA is now saying that lemon juice should be added to tomato sauce before canning ina hot-water-bath canner (google it for more info and for precise amounts to add).
NOTE: Don't try canning without researching about it first. While you can cut corners here and there, and botulism poisoning is extremely rare, it's still better to be safe than sorry. If you don't want to take the risk, you can probably freeze the sauce.