Place your starter mixture on the kitchen counter. After a couple of days, things will start to break down and ferment. Check the starter every few days. Add a bit more flour and clean filtered water to maintain the volume and keep things from getting too dry or thick.
After a few weeks on the kitchen counter, your starter will begin to acquire that familiar fermented sourdough aroma. This aroma only gets better as your starter ages. At this point it is okay to place your starter in the refrigerator. This slows down the ferment, but does not seem to hurt anything.
Be sure to routinely feed and water your starter with a couple of tablespoons of flour and enough filtered water about once per week to maintain a consistent volume in your container.
I usually use about 1/2 cup of starter for each bread recipe.