Combine lemon juice (or vinegar) and milk in a small bowl. Mix well and let sit 5-10 minutes, or until mixture begins to curdle. Use in a recipe that calls for buttermilk!
Notes
If you are looking for a vegan or dairy-free substitute, almond milk will get the job done. In fact, coconut, soy and rice milks all work well, too! Use 1 cup of non-dairy milk and increase the amount of lemon juice/white vinegar to 2 tablespoons.