Day 1: Combine the pumpernickel or whole wheat flour with the cool water in a non-reactive container. Glass, crockery, stainless steel, or food-grade plastic all work fine for this. Make sure the container is large enough to hold your starter as it grows; we recommend at least 1-quart capacity.
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. See "tips," below, for advice about growing starters in a cold house.
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 1/2 cup), and add to the remainder a scant 1 cup King Arthur Unbleached All-Purpose Flour, and 1/2 cup cool water (if your house is warm); or lukewarm water (if it's cold).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 1/2 cup, once it's thoroughly stirred down. Discard any remaining starter. Add a scant 1 cup (4 ounces) King Arthur Unbleached All-Purpose Flour, and 1/2 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.
Day 4: Weigh out 4 ounces starter, and discard any remaining starter. Repeat step #6.
Day 5: Weigh out 4 ounces starter, and discard any remaining starter. Repeat step #6. 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 discarding and feeding every 12 hours on day 6, and day 7, if necessary — as long as it takes to create a vigorous (risen, bubbly) starter. Note: see "tips," below.
Once the starter is ready, give it one last feeding. Discard 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. Hate discarding so much starter? See "tips," below.
Remove however much starter you need for your recipe — typically no more than 8 ounces, about 1 cup. If your recipe calls for more than 1 cup of starter, give it a couple of feedings without discarding, until you've made enough for your recipe plus 4 ounces to keep and feed again.
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. Feed this reserved starter with 1 cup of flour and 1/2 cup water, and let it rest at room temperature for several hours, to get going, before covering it. If you're storing starter in a screw-top jar, screw the top on loosely rather than airtight.
Store starter in the refrigerator, and feed it regularly; we recommend feeding it with a scant 1 cup flour and 1/2 cup water once a week.