HOW TO MAKE BUTTERMILK—Buttermilk improves the flavour and texture of some baked goods, increasing rise and giving them a tangy taste. It is also essential when using baking soda as a raising agent. Buttermilk can be difficult to find in some places so, this substitute, acidified milk, works well for leavening and tenderising, though it won’t taste as tangy. 

1 cup milk
1 tablespoon fresh lemon juice or vinegar

Mix milk and lemon juice in a bowl and leave to stand for 5 minutes, till curdling and clumping occurs. Use in recipe as it is.

You can also use cream of tartar if you have it. It is totally tasteless, just add 1-1/2 teaspoons of cream of tartar to 1 cup of milk.