Here's the recipe:
You will probably eat less of this stuff than I do.  I eat a lot of it because my diet is so limited with my wheat allergy and this stuff is great after running, which I do a lot of.  So you can try making a gallon, and just cut the rest of the ingredients.  Yogurt lasts 3 weeks if you use fresh milk.
 
You need:
-a thermometer that can be used to stir the milk that measures up to 200 degrees - digital ones suck and often break when they get wet, which they will get wet.
-A 4 Gallon stainless steel pot with lid.  It should have a large bottom area to minimize heat loss.
-Tall thickglass jars.  Tall and thick jars retain heat.  I got 2 perfect jars from at the Hadley Flea Market, I have no clue where they come from or what they're used for.  I also use 0.5 gallon milk jars, but they have thin mouths at the top, so you have to break the texture of the yogurt to eat it.
-I also found good jars at Different Drummer's Kitchen in Northampton MA.  The jars you buy must be only glass and a stainless steel lid attachment is OK.  If the lid has other material it is ok, as long as the lid is detachable
-An oven that these jars can fit in!
 
Buy:
3.5 gallons of nonfat milk
3.5 cups of powdered milk at stop and shop, buy the big box for like $18, not the packets
2 Tablespoons of salt
4 Tablespoons of Honey
3 Tablespoons of molasses (optional) - this gives the yogurt bugs minerals to munch on and grow more quickly
1 pound of Trader Joe's plain greek yogurt - (you can use more than this amount if you want)
A deep whisp that can tolerate up to 200 degrees.
Cast Iron pans (totally optional)
 
Heat up the milk slowly and stir frequently.  Don't let it boil or burn.  Use low heat.
When it hits 150 to 170 degrees, turn it off.  Stir in the powdered milk, salt, honey, molasses.
Let it cool to 125 degrees.  I usually put the pot on the porch.  Lately I've left it inside my house with the lid on for free heat.  It just takes more time.
When it is at about 130 degrees, put the jars (with lids off) in your oven.  If you have cast iron pans, put them in the oven.  They hold insane amounts of heat. 
Turn the oven up to 200 degrees for about 10 minutes, then shut the oven off.  The oven stays warm for hours even if it is off.
When it is 125 degrees, stir the yogurt very well into the milk, then pour the milk into glass jars.
Put the jars into the oven.
You can leave the jars there for 4-10 hours.  It is best to turn the oven on for 5 minutes every few hours.  But I usually end up doing this as an overnight thing, so it is done when I wake up.  The yogurt bugs will die if the yogurt gets above 125.  But you can turn the oven up to 200 for 5 minutes without the temperature of the yogurt etting that hot.
 
Put the jars in the fridge when done.  See my webpage below my name for my webpage - I have fruit recipes on my recipes folder.