Combine instant potatoes, sugar, water, and yeast in a covered container. Let the starter sit on a counter for 5 days, stirring daily with a wooden spoon.
On the morning of the fifth day, feed the starter with 3 tablespoons instant potatoes, 3 tablespoons sugar, and 1 cup warm water. In the evening, take out 1 cup of the starter to use in a sourdough recipe. Refrigerate the remaining starter.
Every five days, feed the starter 3 tablespoons instant potatoes, 3 tablespoons sugar and 1 cup water. If starter is to be used in a recipe, let the fed starter rest at room temperature 6 hours before use. If starter is not being used in a recipe, keep refrigerated and discard 1 cup of starter after each feeding.