I live in the San Francisco Bay Area - in Berkeley, CA. Though there is a renowned bakery just two blocks from my house (Acme Bread Co.) and another less than 2 miles away (Semifreddi's), I have never been successful at getting a sourdough starter going from the wild yeast in my own backyard... until I tried using malt extract.
1. Add 1/4 cup of dry malt extract to a solution of 2 cups AP flour and 2 cups water
2. Stir sticky batter together in a large mason jar or other clean glass container - covering with rubber-banded cheesecloth
3. Leave outside in the shade (in my case, under a plum tree full of ripening plums) for 6 hours
4. Bring inside, into kitchen cabinet
5. After active fermentation appears complete (looks a lot like a beer fermenting, only uglier), pour off "beer" that has collected on top, and stir in 1/2 cup AP flour
6. When bubbles have appeared on the surface, remove half of the starter (can be used for bread baking, or just discarded), and replace with 1 cup flour and 1 cup water
7. Starter is now mature. Remove as much as you need for baking and replace with equal parts (by volume) flour and water
I have been using this for extra sour "San Francisco Style" sourdough breads by putting together the dough, giving it 12-24 hours in the fridge, then 12 hours room temperature fermentation, followed by 10-12 kneads, shaping into a boule (the picture shown was today's batch - my first attempt at shaping a bâtard), giving it another 90-120 minutes, before baking in a preheated Dutch Oven (leaving the last 15 minutes for baking uncovered.