Dissolve yeast in 1/2 cup warm water, stirring well; let stand 5 minutes or until bubbly.
Combine remaining water, flour, and sugar in a medium-size, nonmetal bowl; mix well. Add dissolved yeast, and stir well. Cover loosely with plastic wrap or cheesecloth, and let stand in a warm place (80° to 85°) for 72 hours, stirring 2 to 3 times daily. 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, and measure amount of starter needed for recipe. Replenish remaining starter with Starter Food, and return to refrigerator; use within 2 to 11 days, stirring daily.
Repeat procedure for using and replenishing Sourdough Starter.