This can take up to three days. A pinch of yeast can be used to kick-start the process if you get impatient. Once fermentation has begun, you must 'feed' the starter. Equal portions of flour and water are added every three days or so. This fortifies the starter.
Some bakers depend solely on the starter for leavening.
Others use a starter in conjunction with yeast. Bread baked with a starter alone has the most flavor and texture. Bread baked with a starter and yeast does not have the same quality of flavor but it is still very good. Many professional bakers operate this way. A sponge is water and flour with a touch of yeast.