For full disclosure, this recipe/method comes from the book Tartine Bread. If you read our blog entry on bread cookbooks and you wish to make a sourdough for any of the breads in Daniel Leader's books, you will need to purchase those books, Local Breads: Sourdough and Whole-Grain Recipes from Europe's Best Artisan Bakers or Bread Alone: Bold Fresh Loaves from Your Own Hands, because his recipes take very specific amounts of water to grain.
To begin, pour about a half cup of water into a clear container with a lid, then throw in a handful of whole wheat flour and a handful of white flour (you can use only white flour if you wish but the mixture does add to the end results). Mix this together until it is thick and has no lumps (if it is runny, you might want to add more flour). Cover the container with a towel and put it in a dark, cool corner for 2 to 3 days. After these days are over, check the container. You should see bubbles around the sides and on the top of the culture. If there are no bubbles, cover it and put it back into the corner. If you do see bubbles, you should also see a dark crust on the top of the mix. Lift this off with a spoon and discard it. You should now be assaulted by the smell of very stinky cheese and, if you are brave, you will find that it tastes very acidic. Now you have sourdough to be fed.
Throw most of the starter away. Add a splash or two or water and another handful of both whole wheat and white flour and stir them in until it is thick and lump free. Put the lid on the container. Repeat this every 24 hours. You should notice as the week goes along that he volume of your starter will gradually increase. The aroma should become less objectionable too. You must throw a portion of the starter away each time you feed it. The wild yeasts in the air grow on the nutrients in the flour. They also discharge carbon dioxide during this process and this quickly turns to alcohol. Now, it is an interesting thing that this feeding is necessary for the life of the yeasts but that the very carbon dioxide that is released will eventually kill said yeast (alcohol and yeast is a bit of a balancing act. If you get too high a concentration of alcohol, bye bye yeast). As long as you discard most of the mixture, you should not have problems keeping the yeast alive. Once you deem your sourdough ready to use (your sourdough should be almost doubling in volume after adding new flour and water but, if you can't judge this, give it a week and then try to make bread with it), continue feeding it, not forgetting to discard most of it each time, and store it in the refrigerator. If you store it in the refrigerator, a weekly feeding should be sufficient. If you plan to go on vacation for more than a week or two, give it to a kind neighbor with instructions to feed it for you. If it does exhaust itself, just start feeding it again. It will come back. Sourdoughs are intimidating to many people but really, they are quite forgiving.
The night before you want to make bread, add 200 grams of room temperature water, 100 grams of white flour and 100 grams of whole wheat flour to 1 tablespoon of the sourdough starter you have generated. The next morning you will be able to see how your starter did. If it floats in water, it is perfect for making bread. If it sinks straight to the bottom, you will want to give up on making bread that day (or throw in some commercial yeast, if you are impatient) and continue feeding it until it does float.
If you have starter left after mixing your bread, throw it back in with the starter that you have stored in the refrigerator.
And now you are ready to make bread.
Note: This starter will work for both the recipes in Tartine Bread and Josey Baker Bread.