Combine the skim milk and the commercial buttermilk, mixing well. Let stand, covered, with plastic wrap in a warm place (similar to where you would place bread to rise) for 16 to 24 hrs. Or until thickened. Refrigerate in a covered container. Shake well or mix in blender before using. Store in refrigerator for up to 2 weeks. Makes 2 cups. To replenish: Use 2 tbl. Of your buttermilk for commercial buttermilk.