This is how Dads Mom made their buttermilk when he lived at home, this would have been in the 1920's (although they always used fresh raw milk, at the moment I do not have a cow to milk). I never ask Dad why, but he always called this sweet milk (but when mom purchased it from the store he called it buttermilk)?
Start with buttermilk you purchase at the store (make sure it is fresh) and a clean jar.
Add 2/3 cup of buttermilk to the jar and finish filling with fresh whole milk.
Screw the lid on tight and shake, label and store in a dark warm place for 24 hours
After 24 hours if you turn the jar sideways it should coat the glass (like in the picture below) then it is ready and needs to be refrigerated immediately. When you get down to about 2/3 cup of your homemade buttermilk add this to another very clean jar and fill again with whole milk. If you keep doing this you never have to purchase buttermilk again! :)