1. Choose your soffritto base. Onion or garlic are fine, more exotic variations include scalogno or porro.
2. Choose your tomato. Canned, fresh, whatever, just keep in mind that fresh ones may need longer cooking times. As for canned, check that they contain no seasoning at all!
3. Choose your grease. Oil or butter are fine, the standard is olive oil though. It may be hard to find proper olive oil outside of Italy I'm told.
4. Start cooking. Put your oil in a large pan, enough to contain all the pasta you plan to use afterwards. Not too much oil: just enough to cover the pan with a thin layer. Don't start heating the pan.
5. Cut your onion or whatever in small pieces and add them to the oil. Now turn on the heat at a reasonable level. Not too high but not low. Don't touch the onion!
6. When the onion looks a bit browny (not dark brown), add the tomato and lower at minimum the heat. If you have a thermometer, ideally you don't want to cross 60 degrees celsius over all the cooking period. This period can vary between 10 minutes and 60 minutes, it gives different tastes (all good) to the sauce. If you opt for the shortest time, go back at step 5 and at the same time start the next step.
7. Put 1l of water for every 100g of pasta in a pot. Add salt. With experience you'll get the right amount, usually I use about a small fist for two people (160-200g). Heat up the water and wait until boiling.
8. Drop the pasta in the water. Start a chronometer. Almost immediately mix it or otherwise it will stick. Wait a couple of minutes and mix again.
9. Meanwhile the sauce will start bubbling and, depending on your kitchen, you may need to mix it. If you see large discrepancies in texture, definitely mix. Otherwise don't. If it becomes too dry, add some water from the cooking pasta to the sauce.
10. When the chronometer is at cooking_time_on_pasta_packaging - 2 minutes, take a glass of water and fill it with water from the pasta pot. Dry the pasta, and put it in the pan with the sauce. Make the heat level for the pan a bit higher.
11. Cook it until "al dente", that is still a bit hard at the inside, but not completely. If the sauce dries too much (it should, if not turn the heat higher), add the water you kept in the glass. This step is where science stops and art begins: you need to calibrate your taste to your desired results and in turn calibrate water and heating. During all this step, mix your pasta in the same direction continuously. This is called "risottatura". Taste the pasta while cooking often.
11. Take everything off the fire, serve, add parmisan.
Edit: look at maccard comment for water and salt because I don't recall the right quantities. After a while you go by eye.
Edit 2: preventing more comments on oil, that is merely my very limited experience and I'd say, as a rule of thumb (not incontrovertible truth), that if you like your oil alone with bread it is a good oil.
