Warm milk to 90º to 100º. Stir in yogurt.
Pour into a warm 3- to 6-cup container (glass, ceramic, plastic, or stainless steel) with an airtight lid.
Cover and let mixture stand in a warm (80º to 90º) place until it has the consistency of yogurt, 18 to 24 hours; the mixture should be so thick it doesn't flow readily when container is tilted. A single clot may form or smaller curds may be suspended in clear liquid. Stir to mix in any clear liquid. If liquid turns bright pink, discard the batch and start again.
Once a clot forms, add flour and stir untl smooth. Cover airtight and let starter stand in a warm (80º to 90º) place until it is full of bubbles and has a pleasing sour smell, 2 to 5 days. Again, if clear liquid forms, stir to blend mixture, If liquid is pink, discard and start over. To store, cover airtight and refrigerate.
Using and maintaining the starter:
Notes: For most active starter, feed at least once a month. To increase starter supply, in a large container, add as much as 10 cups each of milk and flour to 1 cup of starter. The mixture may need to stand up to 2 days before the clear liquid forms on top.
Use starter at room temperature. To hasten, set container in warm water.
Use and/or feed starter. To feed, replenish each 1 cup starter used with 1 cup warm (90° to 100°) nonfat or low-fat milk and 1 cup all-purpose flour. Stir to mix well.
Cover starter airtight and let stand in a warm (80° to 90°) place until bubbly and sour-smelling, and clear liquid has formed on top, 12 to 24 hours.
Stir before using. Store airtight in the refrigerator.