Mix the flour and yeast together in a container that can hold two quarts. Stir in the water and mix well. 

Cover the container with a dish cloth and let it sit in a warm spot. The dish cloth will let the yeast pass through into the batter. The mixture should bubble as it ferments. 

Stir it once a day and let it sit out for 5 days. Then stir it, cover with plastic wrap, and store it in the refrigerator.

As your starter sits in the refrigerator, a liquid will form on the top. Stir the liquid back into the starter before using.  

Your starter should be fed every other week. To feed it, remove one cup (use it in a baked item, give it to a neighbor, or throw it away) and replace it with 1 cup of water and 1 cup of flour.  Let it sit out for a few hours; then refrigerate.  

When you are ready to cook or bake with your starter, remove it from the refrigerator and pour it into a bowl. Let it warm to room temperature. At this time, mix up equal parts of water and flour to equal the amount of starter you are going to need.

If your sourdough gets too sour, throw all of it away except 1 cup. Add 2 cups of flour and 2 cups of water to it, and let it ferment for a day or so.

If you will not be using your starter for some time, freeze it. Two days before you need to use it, let it defrost. Then feed it and let it ferment for a day.

NOTE:  Because I don't use my starter everyday, I store it, covered, in the refrigerator until ready to use. When I decide I want to use my starter, I remove it from the refrigerator 1 1/2 to 2 days ahead of time and let it sit out overnight. Then feed it with 1 cup flour and 1 cup warm water. Let this sit 8 hours or overnight. It is now ready to use.