This recipe is from Sunset Magazine. I have used it for many years, and it is very reliable. Sunset notes that for the first six months it is best used in recipes with supplementary leavening.
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.
06To store, cover and refrigerate.
07Always bring your starter to room temperature before using it (this takes 4 to 6 hours).
08To 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.
09If 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.