Ok. Here's how to make your own plain yogurt. I did this two days ago and it turned out perfectly--it tastes just like store bought yogurt.
Heat a quart of whole milk in a glass container (such as Kerr jars) in a pot full of water, kind of a make-shift double boiler. You want to heat fairly slowly so it doesn't scald. Heat to 185 degrees to kill all the bacteria.
*Make the level of water about the same height as the level of milk. Thus for a quart of milk, you'll probably want to use two jars instead of just one, so that each is half full. This will heat the milk evenly and avoid the jars from tipping over.
*Do this part with your jar lid off.
Place heated milk on a trivet in the fridge to cool the milk to about 110 degrees. This is the ideal incubating temperature.
*Don't put the glass jars in cold water, as the sudden change from hot to cold will crack the glass jars.
*Again, the jar lid is off.
Next add 2 tbsp. yogurt starter. This can be plain store bought yogurt, yogurt from a previous batch, or freeze- dried yogurt cultures. The idea is to get the live cultures in there. You can also add 1/4 c of Nonfat Dry Milk .
*Homemade yogurt isn't as thick as store-bought yogurt. Name brands stick gelatin in their yogurts to thicken them up. The nonfat dry milk does the same, but is a somewhat healthier way to thicken.
Put the glass jars in a crockpot filled with water (again at the same level up the bottle as the yogurt is), turned to warm. The temp should hover around 100 degrees. Put a towel over the jar(s) and let incubate for 6-10 hours. The longer it sits, the thicker it gets, but also the more tangy. I tend to take it out at the 6.5 hour mark, because it's sweeter that way, and I've taken care of the thickening with the dry milk.
*I used a thermometer in my crock pot to find out how hot it got. I got up to 117 degrees, and my yogurt still turned out wonderfully. I didn't cover my jars, though--that seemed to heat the water up too much.
*This is where I put the jar lid on. I'm not sure if it is required, but that's what I did.
*It will look a little runny at the end of this step. Stick it in the fridge overnight and it will be the same consistency as store-bought yogurt.
*If you want to add flavor, do it right before you eat it (such as frozen fruit or fruit juice, etc). I've read that if you try to mix it in the yogurt earlier, the yogurt just turns runny.