I use a quart jar and pour a half cup of buttermilk into it.
Next step is to finish filling the jar with whole milk.Leave headspace( One inch free at the top).
Now cover this jar with cheesecloth and a rubber band.
Let jar sit out on the counter for three days,stirring each day once.The culture in the buttermilk will not let this spoil.So its ok. On day three get it cold and drink as you wish,keeping it in fridge.
REMEMBER to keep a half cup of this for a new jar of buttermilk--just pour into a nice clean jar and top it off with whole milk and sit on the counter for three days.Keep on rotating as long as you like buttermilk.You never have to buy it again!