1/4 to 1/2 cups buttermilk
3 1/2 to 4 cups milk or reconstituted dry milk powder
leave 1/4 to 1/2 cup commercial or homemake buttermilk in bottom of its container. fill with milk, leaving 1/2" to 1" head space. close container and shake well. let sit in warm place all day or overnight. It will take about 10 hours in winter, 5 to 10 hours in summer. When slightly thickened, refrigerate. save some for the next batch. this buttermilk is very sweet-smelling, higher in quality than store-bought and it tastes much better.
this does not work with the buttermilk powder (i know, i tried). make sure to use approved buttermilk and approved milk.