I've long believed that to prepare any dish well you should know when you begin what the final result should taste like. Forgive me if that sounds obvious, but it's been my experience that not every cook follows this simple maxim. What this means in practice is that following an untried recipe is a hit and miss affair. Recipes can be at best only guides; unless you have a clear idea of where you are going you can end up with a dish a long way from what you intended.

What prompts this musing is that recently I've noticed supermarket shelves increasingly stocked with pre-prepared tomato sauces. The choice is daunting, not only with the myriad of brand names but also with the range of flavours on offer: traditional pasta sauce; with added garlic; with spices and herbs; with mushrooms - every conceivable combination known to marketing man. Faced with such a choice, where do you begin?

At least once you should begin with tomatoes. If you make a tomato sauce just once from scratch, then you'll know what it ought to taste like. Once you know that, you're in a better position to choose from the array of convenience sauces on the shelves. You'll have a base-line from which to rate them.

When I was a small boy visiting my relations in Italy I used to go exploring the narrow, cobbled lanes of Gallinaro. From early morning until lunchtime pots of sugo - tomato sauce - sat simmering on ranges and hobs, slowly boiling down, intensifying the flavours. Some might have a ham bone, others sausages added for taste. These days it's hard to find a sauce made like this; modern Italians go for a quick, light tomato sauce that puts less strain on the digestion.

To make a simple salsa Napolitana begin with tinned plum tomatoes. Good ones should be firm, sweet, deep red, and full-fleshed. There are some shippers who put unripe, orange-tinged tomatoes in cans with lots of water. They should be avoided. Drain the tomatoes and slice them open to remove the seeds. The only excuse for having seeds in a tomato sauce is laziness; they shouldn't be there as they add acidity. Next you should push the tomatoes through a coarse sieve with a wooden spoon, although chopping them finely will do at a pinch.

Take a saucepan and cover the bottom with olive oil. Add some finely chopped garlic and let it fry gently until cooked. Add the sieved or chopped tomatoes, a little salt and pepper, some basil and oregano. Cook for fifteen minutes and it's done. If you time it right you can make this sauce while the water boils and the pasta cooks. The longer you cook it, the more it reduces and the more olive oil you'll need.

Once you're comfortable with this sauce you can use it as a base for more complex sauces. But crucially, you'll know what a tomato sauce ought to taste like.