Step one: Get your starter started

Sourdough bread is fairly unique in the world of baking in that it involves keeping a living thing in your refrigerator.

Think of your sourdough starter as a pet. It is not particularly attractive, as pets go. You can't play with it, nor take it for walks, and it will never learn to fetch, but it is a pet nonetheless, one that you give flour and water to on a regular basis. In return, your pet will give you fresh, homemade bread as often as you want, and it will never chew on your slippers.

The secret to a good sourdough starter is a culture of wild yeast (Saccharomyces exiguus) and bacteria that form a stable symbiotic relationship in a mixture of flour and water. The lactobacillus bacteria, similar to the bacteria that make yogurt and cheese, eat the byproducts of the yeast's digestion process; and produce increasing quantities of lactic acid. The yeast can live in this acidic environment, but not much else can. As the yeast grow and reproduce, they produce carbon dioxide bubbles in the batter, and produce the alcohol that gives their bread its distinctive taste. As long as you feed the starter, they can live for centuries.

Some sourdough bakers recommend against making your own starter if you're new to sourdough baking, but the truth is, it's one of the easiest things in the world. Just mix some flour with some water, and nature will do the rest. Here's how to make your starter:

1. Select a container to keep your starter in. I now keep mine in an empty pickle jar with a lid, but when I first started baking sourdough bread, I used an empty plastic cashews container. You can use a stainless steel mixing bowl if you want, but be careful. Some metals will react to the starter and kill it. A glass or Pyrex container is best.
2. Measure a half-cup of warm water, and pour it into your container. Now measure a half-cup of flour, and stir it in. As a rule of thumb, the starter should be about the same thickness as pancake batter. I made my first starter with whole wheat flour, but any type of flour will work, as long as the flour is unbleached. I've made starters with other flours too, including rye.
3. Keep your starter in a warm place in your kitchen. Every 24 hours, remove half of it, and then stir in another quarter-cup of flour and another quarter-cup of warm water. Why should you remove half the starter? Because as the yeasts grow, they are consuming food throughout starter, and the way to keep them well-fed is to double the size of the starter every 24 hours. If you didn't throw half of it away each time, thereby removing half the yeast culture, then in four days, you'd be adding four cups of flour in order to keep the starter fed. You don't have to throw the discarded starter away, though; you can use it to make some fine waffles or pancakes.
4. Within a few days, you should see air bubbles all through your starter, and you should smell something pleasantly sour, like beery. The starter may even start to look puffy. This is all good. When your starter develops a bubbly froth, it is done. You have produced a sourdough starter.
5. Once your starter has begun to froth, put it in refrigerator, with a lid to keep out odors from your other food. At this point, you will need to feed it only once a week, although you can get away with feeding it less frequently. (I once let mine go for four weeks, with no ill effects.) Your starter is difficult to kill, except by heat. Even starving it is difficult, and freezing will only put it to sleep.

Not all starters are created equal. Some will froth and grow very quickly, and others will take this side of forever. You can game the odds in favor of a more vigorous starter by getting two or three of them going at the same time, and discarding the weakest performer like you would do if you were breeding animals. Split the strongest-performing starter into multiple batches, and repeat the process until you find a starter you like.

After your starter has begun bubbling, mark on the side of the jar how high it reaches just after you feed it. Then, every hour or so thereafter, mark again how high it has risen. This will give you a view of how active your starter is, and how quickly it grows.

Once you have your dream starter, remove a few teaspoons of starter, dry them out, put them in a plastic container, and freeze them. Then, if your active starter goes wrong, you can restart it with one of its frozen cousins.