I have made a lot of rock-hard sourdough bread in the past. I blamed my failures on the air in my house not having enough yeast and the climate not being warm enough for the yeast to grow. Later, I realized it was because of my sourdough starter. Most books on baking sourdough bread say, "When your sourdough starter develops a bubbly froth, it is done", but that statement is missing crucial details for making a successful starter. After endless experimentation, I learned the key to baking successful sourdough bread is knowing when your starter is really ready.
There are many great web sites and books out there that tell you how to make a sourdough starter. The methods are basically same. My method is just one of them, which follows a basic rule: feed your starter once every 12-24 hours.
It is not difficult to make the starter bubbly, but how bubbly does it have to be? You must know when the starter is ready with enough yeast developed to bake successful bread. Most of my failures happened because I thought that the starter was ready when it was not.
When making sourdough starter, you can use white bread flour or whole grain flour. However, if you are a beginner, whole grain flour is easier to make a starter out of.
San Francisco Sourdough Bread