The following specific recipe comes from Super Nutrition for Babies by Katherine Erlich and Kelly Genzlinger:
Option #1: Homemade Whey
Generally, the best lacto-fermentation “starter” is whey from grass-fed, raw (unpasteurized) cow’s milk or yogurt.
1 container raw milk or raw yogurt
Allow the sealed container of milk or yogurt to sit out at room temperature for a few days. It will separate into whey (the yellow liquid you often see on top of yogurt) and milk solids (curds).
Pour the “clabbered” (separated) dairy through several layers of cheesecloth (or a coffee filter) lining a mesh strainer. The yellow whey will filter through, and the curds will be captured in the cheesecoth. The straining often takes 16-24 hours.
Whey lasts 6 months in the refrigerator, sealed in a clean container.
Option #2: Pre-made Culture Starter