The recipe below makes a fairly mild sourdough from a controlled starter. If you'd like to develop a stronger flavor, leave the starter for 24-36 hours instead of 12. You can even leave it longer, but you'll need to "feed" it with a small amount of additional water and flour after the first day. The recipe is a modification of one from Bernard Clayton Jr.'s book, The Complete Book of Breads.
Line 3 mixing bowls or other round containers with a floured, clean dish towel or pastry cloth. Round each of the balls of dough in one of the prepared containers. Choose containers for this rising that leave several inches above the ball of dough for rising. Cover with plastic wrap, and leave undisturbed at 70° for 3-4 hours.
About 30 minutes before baking, preheat the oven to 500°F., and line the upper rack of the oven with quarry tiles. The oven racks should be at the lower and upper third of the oven. Put a pan of water on the bottom rack of the oven. Gently invert the loaves onto a piece of parchment paper on a pizza peel. Don't worry that there will be flour residue on the top of the loaves, this adds a more rustic and toasted flavor to the crust. With a paring knife, cut 1/4 inch slashes in a cross hatch pattern on the loaves. Carefully slide the bread (on the parchment paper) onto the hot tiles. As the bread bakes, the parchment paper will get very dark, but don't worry. It's a good way to place the breads on the tiles and to keep them from sticking to either the peel or the tiles. Bake 30 minutes or until the loaf sounds hollow when tapped on the bottom. Cool on racks for at least 15 minutes before slicing.
This is a fairly dense bread, and it keeps well for a few days, wrapped tightly at room temperature. To freshen the bread, warm for about 10 minutes at 350°F. If you'd like to freeze this bread, double-wrap it and freeze for a few weeks. It does not keep well for longer periods in the freezer; the crust tends to separate from the inside.