In a large bowl mix the yeast with the water and allow to proof for about 10 minutes. If the yeast doesn't start to puff up and bubble, discarded and start over with fresh yeast.
Sprinkle the flour into the yeast mixture and beat into a thick batter. The batter doesn't have to be too smooth as the action of the yeast will break down any lumps. Cover the bowl with plastic wrap and let stand in a warm (80-85 F or 25-30 C) place for about 24 hours, or until it is nice and frothy. You can use the starter right away or let it age for up to 3 days longer to develope a richer flavor. Store starter in fridge between uses and use or replenish every two weeks.
You can use any flour or combination of flours to make the starter; some rye and buckwheat added to all-purpose makes a nice blend.
To replenish the starter: mix equal parts of water and flour (about a cup of each is good) and add to the starter at least every two weeks. Let the starter age in a warm place for 6 to 24 hours, then use or return to the fridge.