In a large glass or ceramic bowl, combine 1 cup of the milk, 1 cup of the flour, the sugar and the dissolved yeast. Using a non- metal spoon, beat until smooth and creamy. Stir in the remaining 1 cup milk and 1 cup flour. Cover with a clean kitchen towel and set in a moderately warm spot (about 70F.) Let stand for 24 hours, at which point the batter should smell pleasantly sour.
Transfer to a tightly covered glass or plastic container and place in the refrigerator. Stir with a nonmetal spoon once a day for 5 days.
To use the starter, measure out what you need and let it come to room temperature. Replenish it with equal parts of flour and milk, plus half a part of sugar. (For example, if you remove 1 cup of starter, stir in 1/2 cup flour, 1/2 cup milk, and 1/4 cup sugar.) The starter may be stored in the refrigerator indefinitely, but flavor develops best when batter is used and replenished often.