Ingredients: 1 quart/liter of milk and one-fourth cup/250 ml. plain yogurt
Quantities in this recipe are approximate. What works depends largely on the potency of the yogurt you use, and you'll only know that by trying it out. I used Chobani Greek Yogurt, but many brands will do as long as they contain active cultures--the one I used has five different kinds of yogurt bacteria--and NO THICKENERS such as starch, gelatin, pectin, guar gum, etc. etc. If there are enough yogurt bacteria doing their job, the yogurt will be reasonably thick on its own. If they've used a thickener, chances are there aren't enough bacteria in there.
Scald the milk -- i.e. heat it up, but take it off the stove just before it climbs the sides of the pan and makes a huge mess.
Pour the milk into a glass or ceramic container with a lid. (Don't put the lid on yet – that's for later.) Cool the milk to around 110-115 degrees F. It helps to use a thermometer the first few times until you can tell the approximate temperature by dipping your finger in. My little finger works best, perhaps because it's less desensitized than the others.
Stir the yogurt into the milk. Cover with the lid, wrap in a blanket (two layers is good), and put it somewhere warm. All I mean by warm is somewhere not cold. In my house in the summer, anyplace will do because I don't have good A/C. In the winter, I'd put it somewhere sheltered in the warmest room of the house.
Open it 24 hours later, and you should have yogurt. If you don't, or if the yogurt is too liquid… hmm. You might want to try adding more yogurt next time, or try a different brand, or keep the container in a warmer place. Once you've unwrapped the yogurt, store it in the fridge.
Next time, use some of your homemade yogurt as a starter. It should work fine, but again, results vary. Over the course of time, your batches may become stronger and more sour, and even develop quite a bite, so if that doesn't appeal to you, you can always start over with a milder yogurt from the store.