In a medium saucepan, cover potatoes with water. Cook over medium heat until tender, about 20 minutes. Remove potatoes and reserve for other use. Let cooking water cool to lukewarm.
Mix flour and 1 cup potato water. Set aside in bowl with cloth or wax paper to cover. Mixture does not have to be mixed smooth: the batter will work out any lumps as it sits. Stir once or twice a day until it smells right. It will have a nice sourdough smell to it after about 4 or 5 days, depending on the temperature of your kitchen. You can add a teaspoon of sugar to quicken the process. If you want to speed the process even more you can add a package of yeast to the water before adding the flour. Use as directed in your favorite Sourdough Bread recipe.