At least 1/2 cup of starter must be used (or discarded) every two weeks. Replenish with equal amounts of flour and water equivalent to the amount of starter used (i.e., 1/2 cup starter is replenished with 1/2 cup water added to 1/2 cup flour and the dry milk).
In plastic container with lid, combine water and milk, stir until dissolved. Add yogurt, cover and put in warm place 12-24 hours (until the consistency of yogurt).
With a plastic or wooden spoon, add the flour and stir until smooth. Cover and put in warm place until bubbly and slightly sour (2-4 days). Refrigerate in plastic container.
Combine flour, water and milk in small bowl and mix well. Add to active starter. Cover and keep at 80-90° until bubbly. Refrigerate until needed.