Soften yeast in 1/2 cup warm water.
Stir in flour, 2 cups warm water, and sugar.
Beat till smooth.
Cover with cheesecloth; let stand at room temperature till bubbly, 5 to 10 days, stirring 2 or 3 times a day.
Store Starter, covered, in the refrigerator.
To use, bring desired amount to room temperature.