Combine the flour, water and yeast in a tall 2 or 3-quart clear plastic container with a lid. Stir well to make a thick, soft dough. The exact consistency of the dough will vary with the brand of flour, but don't add more flour or water at this point to adjust the texture. Scrape down the sides with a rubber spatula, cover tightly with lid, and let stand in a moderate (about 70 degrees) place for about 24 hours.
The chef will now have almost doubled in volume. You will see tiny bubbles on the surface, and you might notice a slight musty smell. Add the flour and water to the mixture and stir vigorously to distribute the fresh ingredients and add fresh oxygen to the chef. The texture will still be like a soft dough. You may add a little more flour or water to make this texture, if necessary. Scrape down the sides, cover and place in a moderate (70 degrees), draft-free place for 24 hours.
The chef will now have the texture of a thick batter. It will have almost doubled in volume and be quite bubbly. Taste it! It will have a pronounced musty, but not bitter, flavor. The scent will be aggressively vinegary -- that's fine.
Hold the container and notice the network of tiny bubbles throughout the chef. Add the flour and water, and stir well to make a thick batter. (You may have to add a little more water if your flour's absorption level is high.) Lift up the spoon slowly and notice the texture of the chef. It will attach itself to the spoon in sticky, gummy strands. With a marker pen, mark the level of the chef on the side of the container. Scrape down the sides, cover tightly, and let stand in a moderate (70 degrees) draft-free place for 24 hours. The chef may rise and fall, but as long as It doubles at some point during this period, it's fine. The chef is now ready to transform into levain. If you don't want to make the levain immediately, the chef can be refrigerated for up to 3 days. Learn more about the chef, including how to nourish and maintain it, in Dan Lieder's book "Bread Alone".