Let sit for 10-15 minutes until quite chunky -- this will happen quicker if your milk is room temperature or warm. (*Have instant homemade buttermilk by microwaving your milk and lemon juice for 45 seconds on high, then stir! No waiting)
Whisk the milk to smooth it -- it should be thick and creamy once combined.
Use in your favorite recipes or refrigerate up to 1 week before using -- shake or stir well before using!