Add 1 tbsp of fresh lemon juice or white vinegar, and stir.
Let the mixture sit for about 5 minutes.
Stir again and use in your recipe as required.
Option #2:
Combine the sour cream (or yogurt) and water.
Use in your recipe as required.
Notes
The lemon/milk mixture will curdle slightly and this is what it's supposed to do. Also keep in mind that this mixture won't be as thick as true buttermilk, but for baking purposes, it works beautifully.