In large non-metallic bowl, mix together dry yeast, warm water, and all purpose flour and cover loosely.
Leave in a warm place to ferment, 4 to 8 days. Depending on temperature and humidity of kitchen, times may vary. Place on cookie sheet in case of overflow. Check on occasionally.
When mixture is bubbly and has a pleasant sour smell, it is ready to use. If mixture has a pink, orange, or any other strange color tinge to it, THROW IT OUT! and start over. Keep it in the refrigerator, covered until ready to bake.
When you use starter to bake, always replace with equal amounts of a flour and water mixture with a pinch of sugar. So, if you remove 1 cup starter, replace with 1 cup water and 1 cup flour. Mix well and leave out on the counter until bubbly again, then refrigerate. If a clear to light brown liquid has accumulated on top, don't worry, this is an alcohol base liquid that occurs with fermentation. Just stir this back into the starter, the alcohol bakes off and that wonderful sourdough flavor remains! Sourdough starters improve with age, they used to be passed down generation to generation!