The Process in a Nutshell
A sourdough starter—also called a culture or levain—is a mixture of flour, water, and microorganisms that flavors and leavens bread. Wild yeasts and bacteria are naturally present on wheat kernels and on flour ground from them, but it takes time and proper care for them to multiply and transform the initial mixture into a bubbly, boozy-scented culture that can leaven bread.
First, you mix flour and water and let the mixture sit for a day or longer until the dormant microorganisms on the flour wake up. After that, you “refresh” (or “feed”) the nascent culture first on a daily and later on a twice‑daily basis by moving a portion of it to a new mixture of flour and water and discarding the remainder. (Don’t worry about waste—you can save the leftover portion as backup and for use in other applications.) After a few weeks, the starter will have built up a sufficient amount of the appropriate yeasts and bacteria it needs, and it can be used in baking.
One important thing to note, though, is that the time frame of each step is approximate. How quickly your starter moves from one stage to the next will depend on the flour you're using to refresh it and how hospitable the environment is for yeast and bacteria activity. Let visual and olfactory cues be your guide and use the day count as a reference.