If insects or other critters are a potential problem, cover the bowl with cheesecloth. Stir the mixture vigorously at least twice every 24 hours. In 2 or 3 days, bubbles should appear on the surface as the first indication that you have been successful. You may need to repeat this additional feeding at 12-hour intervals for several days. When you capture a yeast that is active enough to be useful, it will form a layer of foam 1 to 2 inches deep.
Culture is ready to use in the following first proof.
In a 4-quart mixing bowl, combine the culture, flour and warm water. Mixture need not be lump-free. Let rise for about 12 hours. Measure out the 4 cups of culture called for in the challah recipe. Place the remaining culture in an airtight container and refrigerate to reserve for another sourdough bread recipe.