Mix the milk and white vinegar until it is well combined
Let the mixture stand for two minutes until the milk begins to curdle
If you are using US Imperial measurements the easy way to remember the quantity of each ingredient is "1 of tablespoon of this 1 cup of that" giving you 1 tbsp of white vinegar to 1 cup of milk.
You can substitute an equivalent amount of fresh squeezed lemon juice in place of the white vinegar. This will give you a completely different flavour profile, with a hint of citrus to your recipe. And sometimes, I have just run out of white vinegar.
You can store your buttermilk in the refrigerator for several days.
This recipe easily doubles up. But interestingly it doesn't divide too well once you drop to about 115 ml milk and 10 ml of vinegar. If I only need 50 ml of buttermilk I will go ahead and make up 120 ml, use what I need, and dispose of the remainder. For some reason there's never quite enough acidity in the smaller amount of vinegar to cause the milk to curdle properly.