Fill a 3 to 6-cup container with hot water and let it stand. In a pan, heat the milk to 90-100 deg.F. on thermometer. Remove from heat and stir in yogurt.
Drain water from container, wipe dry, and pour in milk-yogurt mixture. Cover tightly; if using a screw-top jar with a metal lid, place a double layer of plastic wrap over mouth of jar before screwing on lid. Let stand in a warm place (80-90 deg F.).
After 18-24 hours, starter should be about the consistency of yogurt - a curd forms and mixture doesn't flow readily when container is slightly tilted. (It may also form smaller curds suspended in clear liquid). If some clear liquid has risen to top of milk, simply stir it back in . However, if liquid has turned light pink, milk is beginning to break down - discard and start again.
After curd has formed, gradually stir in flour until smooth. Cover tightly and let stand in a warm place (80-90 deg F.) until mixture is full of bubbles and has a good sour smell (2-5 days).
If clear liquid forms during this time, stir it back into starter. But if liquid is pink, spoon out and discard all but 1/4 cup of starter, then blend in a mixture of 1 cup each warm skim milk or low-fat milk (90-100 deg F.) and flour. Cover tightly and let stand again in a warm place until bubbly and sour smelling.
To store, cover and refrigerate.
Always bring your starter to room temperature before using it (this takes 4 to 6 hours).
To maintain an ample supply, replenish your starter every time you use it with equal amounts of warm milk and flour. Use the same type of milk and flour to maintain consistency. Cover tightly and let stand in a warm place for several hours or until bubbly; then cover and refrigerate.
If you bake regularly, your starter will stay lively. If you don't bake often, it's best to discard about half your starter and replenish it with warm milk and flour about every 2 months.