Dissolve yeast in 1/4 cup warm water. Add sugar, vinegar, salt, all purpose flour. Add remaining water until a creamy batter is formed. Place in a glass bowl,cover and let sit until it starts to ferment (about 3 days). It will take on a powerful boozy smell. Stir again until creamy and measure out what is called for in the recipe. Replenish starter with equal amounts of flour and water. Store in the fridge and bring to room temp before using. (It says to allow to ferment for one week between uses but I don't. I do let it sit out overnight after I feed it.) This starter took about 1 1/2 months to become really sour.