Here's an easy sourdough starter to get you going. In a matter of days you will be baking some fantastic breads and cakes. When you start baking with this "secret weapon," you will see why some people keep starters going for years!
In a large vessel (I use a bulk-size mayo jar), soften the yeast in 1/2 cup warm water (not too hot or the yeast will die--test on your wrist).
Stir in 2 cups warm water, the flour and the sugar. Beat until smooth, cover with cheesecloth, and let stand at room temperature 5-10 days, stirring 2 or 3 times a day, until mixture begins to bubble and ferment. Time required for fermentation depends on the room temperature: if the room is warm, the process will take the shorter amount of time; if the room is cool, a few more days may be required.
When the starter reaches fermentation, cover tightly and refrigerate until ready to use for baking.
To keep the starter going: after using some starter, add 3/4 cup water, 3/4 cup flour, and 1 tsp sugar to remaining starter. Let stand at room temperature until bubbly, at least 1 day. Re-cover and refrigerate. If the starter is not used within 10 days, add 1 tsp sugar. Add 1 tsp sugar every 10 days until starter is used for a recipe.