Add the warm water to a crock jar that holds at least 1 gallon. This will prevent the starter from frothing over while it’s setting up.
Whisk in the yeast and sugar and let sit for 1 minute.
Slowly whisk in the flour. You want the consistency like a thin pancake batter, so adjust the water and/or flour if necessary.
Drop the potato pieces into the bottom of the crock jar. Cover with a tea towel and let sit on the counter for at least 12 hours, stirring halfway through. You can let the starter sit for 24 hours for a more sour flavor.
Before using the starter in a recipe, whisk it briskly until smooth.