Get out a liquid measuring cup that will hold at least one cup of liquid. Add the tablespoon of vinegar to the measuring cup. Fill the rest of the way with milk.
Let it sit for 5 minutes to begin curdling. Then use as you need in the recipe.
Use sour milk whenever a recipe calls for buttermilk that you do not have. You can add sour milk in place of regular milk in muffins, pancakes, waffles, or quickbreads to add extra lift to them.
Notes
Variations: ½ Tablespoon Vinegar for ½ cup of Milk2 teaspoon Vinegar for 2/3 c of MilkThe acid in sour milk helps with the rise reaction with baking powder and baking soda. This is part of the reason why sourdough discard helps make these quickbreads fluffier. The discard is acidic as well.