Fill a 3 to 6-cup glass, ceramic, plastic, or stainless steel container with hot water and let stand. In a pan, heat skim melk to 90 to 100 degrees on a thermometer. Remove from heat and stir in yogurt. Drain water from warmed container, wipe dry, and pour in milk 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. After 18 to 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 during this time, 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 the flour into starter until smooth. Cover tightly and let stand in a warm place (80 to 90 degrees) until mixture is full of bubbles and has a good sour smell (2 to 5 days). If clear liquid forms during this time, stir it back into starter. If liquid is pink, spoon out & discard all but ¼ cup starter; then blend in a mixture of 1 cup each warm skim milk or low-fat milk (90 to 100 degrees) and flour. Cover tightly and let stand again in a warm place until bubbly and sour smelling. To store, cover and refrigerate. Makes about 1½ cups.