Sourdough is a culture of yeasts and beneficial bacteria that occur naturally in flour. Making sourdough bread is incredibly simple. Ordinary people have been doing it for thousands of years. You take some starter, refresh it with several times its own weight of fresh flour and water and let this ferment for some hours until the yeast population has grown (the mixture will bubble or increase in volume). You use most of this dough to make bread by adding more flour, water and salt, and keep a little bit back as your starter for the next batch of bread. It really is that easy.