From Hugh Amano
To capture airborne yeast, combine a couple of tablespoons of flour with a couple of tablespoons of water. I use filtered water, incidentally–have you smelled tap water lately? There is so much chlorine in it, I fear it won’t give the tiny yeast a fighting chance to breed. So, knead the dough a bit, then put it in a bowl, cover it with a wet towel, and let it hang out. After a couple of days, there will be a bit of a crust on the dough; peel it off and discard it. There should be some evidence, however slight, of yeast production in the form of tiny bubbles or holes in the dough.
Refresh the starter by doubling the amount of flour and water used previously and repeat the process, again checking progress in a couple of days. Repeat again. By the third refreshment, there should be ample evidence of the yeast.
At this point, you should have a small bit of starter, weighing roughly half a pound or so, depending on how much crust had to be thrown out. Add about 8 ounces of flour, and enough water to make a firm dough that is slightly tacky to the touch, but not sticky. Knead the dough for about 15 minutes. Let rise until about double in size. Knead in about 2 tsp. of salt, and shape bread as desired. When doubled in size, bake in a 425 degree oven until done, maybe 20-25 minutes–it’ll sound hollow when tapped, or the internal temperature will be around 190-200 degrees.