Dissolve yeast and sugar in 1 cup warm water in a medium-size nonmetal bowl, stirring well. Let stand 5 minutes or until bubbly. Gradually add remaining warm water and flour; mix well. Cover loosely with plastic wrap or cheesecloth, and let stand in a warm place (85°), free from drafts, 10 to 12 hours. Place fermented mixture in refrigerator, and stir daily; use within 11 days.
To use, let Sourdough Starter stand at room temperature at least 1 hour. Stir well; measure amount of starter needed for recipe, reserving 1/2 cup.
Add 2 cups all-purpose flour and 2 cups warm water (105° to 115°) to reserved starter; mix well and refrigerate.
Repeat this procedure for using and replenishing Sourdough Starter as needed.