DIY Buttermilk: Culture Begins at Home

2 tablespoons buttermilk (store-bought or activated dried starter)
2 cups milk (whole, 2%, or skim, depending on your nutritional needs and preferences)

Directions

1. In a mason jar or other glass container, thoroughly mix the starter and milk. Cover with a coffee filter or piece of cheese cloth (do not seal tightly with a lid) and leave to culture out of drafts at a warm room temperature (between 70-78°F is recommended) until milk has clabbered (10-24 hours).

2. To test if the milk has thickened, tip the jar slightly. It should move away from the wall of the jar as a single mass. Just as with yogurt making, once the milk sets, it will get more tart the longer you allow the culturing to continue.

3. Refrigerate to halt culturing for at least six hours. Stir before using.

http://www.seriouseats.com/recipes/2012/11/diy-cultured-buttermilk-recipe.html