For every 1 cup of milk add 1 tbl lemon juice or vinegar. I use lemon juice. Stir well and let see for at least 2 min. Milk will get clumpy.
I am trying a new way to make buttermilk. You do have to buy some, but you should only have to do it once, then you will use what you have to make more.
Container to store it in
I did quite a bit of research on the best way to do this. I decided to make mine in quart jars, even though we will go through this buttermilk pretty quick, quart jars will be easy to wash and reuse.
Clean your container you want to put it in. The ratio you want to use when making your own cultured buttermilk in 1 cup buttermilk to three cups milk. You can use powdered milk for this too, but the buttermilk wont be as rich.
Add one cup buttermilk into your container and then add three cups milk. Close lid, shake. Then place milk in warm place for 24 hrs to culturize. The milk should curdle within 48hrs or your buttermilk cutltures are probley dead. If this is the case you can still use it in your baking, but it won't make any more cultured buttermilk for you and you will have to get some more buttermilk to start another cultured batch. After the milk has culturized place in fridge and it is ready to use whenever you need it.
Always save a cup of the buttermilk you made so you can make more. After about three weeks the cultures start to get weaker so it is a good idea to make more before then.