Heat milk to 100F on thermometer. Remove from heat and stir in yoghurt. Pour into scalded glass jar or bowl, cover with plastic and place in a warm location for 18 hrs. Consistency will be like thin yoghurt. Stir in flour until well blended, cover again with plastic and pierce with fork to release gases. Place in a warm draft-free location at an even 85F for 2 days; stir several times each day. It should have a strong sourdough smell and show bubbles. Refrigerate until ready to use. When replenishing starter, add lukewarm milk instead of water.