All sourdough recipes require a sourdough sponge.

Start the night before. Let the starter you made sometime in the past.  Add it to a plastic, glass, or crockery medium mixing bowl (Don’t use metal, even stainless steel, for the sponge bowl).  Add the flour and water. Cover it and put it in the oven with the light on. The light will keep the oven warm, but not hot. Leave in the oven overnight.

The next morning the mixture should have doubled in size. It will have a pleasant yeasty odor and be covered with bubbles. Blend it together with a fork. Before using in a recipe, take 1 cup of this mixture to use as sourdough starter for your next recipe.  Put it in a crock, seal it, and put it in your refrigerator.