In a 1-quart pan over medium heat, heat milk to 90-100 degrees. Remove from heat and stir in yogurt. Place in a warm 3 to 6 cup glass, ceramic, plastic or stainless steel container with a tight lid. Cover and let stand in a warm place until mixture is the consistency of yogurt and a curd has formed. (You can test the mixture by tilting the container. If the mixture doesn't flow freely when the container is tilted, it's ready.) The curding process takes about 18 to 24 hours. You'll need to check it periodically. If some clear liquid rises to the top of the milk, stir it back in. NOTE: If the liquid turns pink, discard the batch and start again. Once the curd has formed, add the flour and stir until smooth. Cover again and let stand in a warm place until the mixture is full of bubbles and has a good sour smell. This should take about 2 to 5 days. Again, if some clear liquid rises to the top stir it back in. If the liquid turns pink, discard the batch and start again. When the mixture is ready, you can keep it covered and store it in the refrigerator until you need it. The recipe makes about 1 1/3 cup starter.