Day 1: Combine the flour with the cool water in a non-reactive container. Glass, crockery, stainless steel, or food-grade plastic all work fine for this.

Stir everything together thoroughly; make sure there's no dry flour anywhere. Cover the container loosely and let the mixture sit at warm room temperature (about 70°F) for 24 hours.

Day 2: You may see no activity at all in the first 24 hours, or you may see a bit of growth or bubbling. Either way, discard half the starter (4 ounces, about a half-cup), and add to the remainder a scant 1 cup flour, and half a cup of water.

Mix well, cover, and let the mixture rest at room temperature for 24 hours.

Day 3: By the third day, you'll likely see some activity — bubbling; a fresh, fruity aroma, and some evidence of expansion. It's now time to begin two feedings daily, as evenly spaced as your schedule allows. For each feeding, weigh out 4 ounces starter; this will be a generous half-cup, once it's thoroughly stirred down. Discard any remaining starter.

Add a scant 1 cup (4 ounces) flour and a half cup water to the 4 ounces starter. Mix the starter, flour, and water, cover, and let the mixture rest at room temperature for approximately 12 hours before repeating.

Days 4 and 5: Repeat the instructions for day 3. By the end of day 5, the starter should have at least doubled in volume. You'll see lots of bubbles; there may be some little "rivulets" on the surface, full of finer bubbles. Also, the starter should have a tangy aroma—pleasingly acidic, but not overpowering. If your starter hasn't risen much and isn't showing lots of bubbles, repeat the day 3 instructions for as many days as necessary to create a vigorous (risen, bubbly) starter.

Once the starter is ready, give it one last feeding. Pour off all but 4 ounces (a generous 1/2 cup). Feed as usual. Let the starter rest at room temperature for 6 to 8 hours; it should be active, with bubbles breaking the surface.

Remove however much starter you need for your recipe (no more than 8 ounces, about 1 cup); and transfer the remaining 4 ounces of starter to its permanent home: a crock, jar, or whatever you'd like to store it in long-term. Store this starter in the refrigerator, and feed it regularly; King Arthur Flour recommends feeding it with a scant 1 cup flour and 1/2 cup water once a week.