I've been making all of our yogurt for the last several months, with great success. I calculate that I'm saving at least $700 a year, which is a little alarming. We were eating three single-serving cups of Fage yogurt a day. Lots of money, and lots of plastic trash (my grocery store only offers non-fat Fage in the bigger container. If I want fat, I have to buy the small).
So here's my super-simple method. I use an enameled cast-iron pot (it's an off-brand Le Creuset) and pour an entire gallon of organic whole milk in it. I put it over a medium heat and set the timer for 25 minutes. Meanwhile I get out my previous batch of yogurt and remove a heaping spoonful of it, and place it in a glass measuring cup with a spout, and leave it out on the counter.
When the milk has started to steam (be sure and jiggle the pot and get down at eye level to see if it makes a little steam), I turn off the flame and set the timer for another 25 minutes. When the timer buzzes, I stick my clean finger in the milk. If it's too hot for comfort, I give it another ten minutes of cooling time. If, however, I can leave my finger in for three seconds, I move to the next step.
Which is to ladle out a cup of hot milk and put it in the glass measuring cup with my spoonful of yogurt. I stir this gently with a spoon, just breaking the yogurt up into small pieces, then pour all that back into the big pot and swirl it around a little.
Then the lid goes on the pot, the pot gets set in my Crock-Pot over an inch of water set to the "Keep Warm" setting, and an old bath towel goes over the whole thing. I leave it all day (or all night). Ten to twelve hours.
At the end of that time, I put my big colander in my stockpot, and line the colander with a large thin dishtowel. I dump all the yogurt in, and refrigerate it. In another ten to twelve hours, I have a large quantity of unbelievably delicious Greek yogurt. Very firm, tart, and thrifty.