I like this way to make yogurt:
Buy yogurt to use for culture. I like Seven Stars or Fage.
Freeze in ice cube trays filling each hole half full. (Store in freezer) It doesn't take much at all to culture a quart.
I have a small “Playmate” cooler that holds two wide mouth quart jars.
I measure out two quarts of organic, raw milk into my saucepan and heat it to 180 degrees (I like thicker yogurt, so I use this temp.) I’ve gotten the heating down to a science. Two cold quarts takes 23 minutes on my medium heat to come to 180. It's a shame to heat this lovely raw milk, but to get the texture I want, I kill the good bacteria and enzymes, and re-introduce a yogurt culture. Using organic un-homogenized pasteurized milk is the next best choice.
Cool milk to 110 and pour into jars. You can pour hot milk into jars slowly and let cool there.
Update: I'm currently cooling the milk to 118 degrees and like the taste better.
When 110 (update: 118), drop one tiny cube into each jar, screw on lid and put in cooler. I found that stirring is not at all necessary for me. I think the culture likes to make its way down the jar kind of like growing roots. It always works.
Fill cooler to jar necks with hot tap water. Don't let the water touch the lids because the jar will suck in water as it cools which I don't want.
Close and wait overnight. In the morning I have perfect yogurt and best of all for me is that I used equipment that is also used for everything else. No single use stuff involved which makes it a lot more fun for me. It feels creative and when I get up in the morning I think “oh boy, I get to check my yogurt” and it’s always thick.
After I developed this system, it was simple to get it right every time. It reads complicated, but it is not.