Measure the vinegar or lemon juice into your measuring cup and then fill it the rest of the way with your milk (so you'll actually need a tablespoon less of milk than a full cup because that 1 tablespoon is your vinegar or lemon juice).
Gently stir the mixture together.
Let mixture sit for 5 minutes.
Use in your recipe in place of the buttermilk called for.
Notes
Here are the amounts of milk and vinegar you'll need for common recipe amounts of buttermilk.
Milk
Vinegar or Lemon Juice
¾ cup
2 ¼ teaspoons
⅔ cup
2 teaspoons
½ cup
½ tablespoon OR 1 ½ teaspoons
⅓ cup
1 teaspoon
¼ cup
¾ teaspoon
Another method is to whisk 1/4 cup milk into 3/4 cup plain yogurt. This will create a nice and thick buttermilk substitute.
You can also use milk and sour cream as a buttermilk substitute. Whisk together 3/4 cup sour cream and 1/4 cup milk until you get a thicker consistency that resembles buttermilk.