1/2 gallon milk (I really prefer 2% or whole milk, for a thicker yogurt.)
1 cup sugar (optional)
1 Tbsp vanilla extract (optional)
1 6-8 oz package plain store-bought yogurt (must read "contains active cultures")
You'll also need:
Canning jars or other containers
A heat source (for incubating the yogurt)
* Note: If you just want to make plain yogurt, skip the sugar and vanilla extract, and just use the milk. This is the only way I make yogurt now! I prefer adding my flavorings in the form of whole fruit, granola, etc.
Step 1: Heat the milk
If you want plain yogurt, skip the sugar but still heat the milk.
Step 2: Cool it down.
Once the milk has cooled to 115 degrees, add the vanilla extract and your purchased yogurt, stirring it in well with a sanitized spoon or rubber spatula. It's crucial that you maintain sanitation at all points in this process!
Again, for plain yogurt, skip the vanilla.