Sprinkle dried culture over top of milk, and let hydrate for 60 seconds
Stir for another 1-3 minutes to throughly combine
If using fresh buttermilk as the starter, it can be helpful to whisk one or two cups of milk with starter, and then stir that combined, thinner mixture into the rest of the milk.
Pour into a clean jar, and cover loosely with a cloth, or just set the lid on to without screwing it down
Put jar in a place where it won’t be disturbed, and let incubate for about 24 hours, until thick and tangy.
When the buttermilk is ready, it will probably not be pourable. You will need to stir or shake the buttermilk before pouring it. I usually don’t worry about that and just dip it out with a spoon or ladle, and stir it in the measuring cup.
To store, place in the fridge for up to 2-3 weeks. After this, I recommend a new batch, as the bacteria will have grown quite a bit, making the buttermilk quite strong.