To Make the starter:

Mix the yeast and 1 cup of the warm water in a small bowl. Put into a plastic container, seal, and refrigerate for 3-5 days.

To Make the Starter Feed:

Combine the sugar, potatoes and remaining 1 cup of warm water in a small bowl and stir into starter. Cover loosely (to allow some of the pressure to esacpe as the gases build) and let stand at room temprature for 5-12 hours. The mixture will be bubbly.

Take out 1 cup to make bread and loosely cover the starter and return it to the refrigerator. Feed again after 3-5 days. If not making the bread after feeding the starter, throw away 1 cup to avoid depleting the starter.