Mix together exact amounts of water and bread flour (Chef John likes to use whole grain spelt flour and white bread flour). Cover, let stand at room temperature, and the wild yeast that the starter attracts will begin to ferment. One day later, add more flour and water. Remove half and discard, then add flour and water, and continue the process for a week. Next, run a test by discarding and feeding, then pouring in jar to see if the starter doubles in size. If it doubles, your starter is ready to become sourdough bread! Get the recipe for Chef John's Sourdough Starter.