6-8 oz cultured buttermilk, (make sure it says cultured and is not out of date;check date on carton).Place 3 cups whole milk(store bought works),in clean 1 quart jar with a secure lid, (such as canning jar).
Add bacterial starter of 6-8 oz of active fresh cultured buttermilk to qt jar. Use 6 oz if certain of the freshness of the starter, (a ratio of about one part starter plus 4 parts milk). When in doubt, use a full cup of buttermilk as starter, (a ratio of 1 part starter plus 3 parts milk). Fill jar with fresh milk mix. Screw lid on securely, shake to mix thoroughly. Label with date.
Let sit in warm room till clabbered. It SHOULD be thickened in 24 hours. If it takes longer than 36 hours, the starter was no longer active. The buttermilk may, or may not be tasty if it took longer than 36 hours. (If in doubt it may still be used for baking).
However, 24 hours later (at room temperature) the bacteria should have firmented the milk. When finished, the finished and thickened buttermilk coats the glass. The finished milk should be refrigerated, it keeps easily for several weeks.