In a 2-cup measure mix together the water (distilled), flour, salt and sugar. Add the potato sufficient to make a full two cups.
Pour the mixture into a widemouth glass jar that will hold at least 1 quart. Place a cheesecloth over the container and allow to set in a warm place for 24 hours. Stir and cover tightly with plastic wrap. The mixture will become light and foamy in 2 or 3 days. Stir down each day.
Pour the fermented starter into a glass jar fitted with a tight lid, and place in the refrigerator. In 3 or 4 days when a clear liquid collects on top it is ripened enough for use.
Care for Starters
A starter should be used at least once a week. If not stir it down after 3-4 weeks, spoon out, and discard half of it and replenish the remainder with equal parts flour and water. When replenishing the starter allow it to set out for about 1-2 days before returning it to the refrigerator.