Yogurt

Bring a gallon of whole milk to 90°C (194°F). Do this in a large stock pot so that the milk doesn’t boil over. Get a thermometer with an alarm.
Turn off heat and let it cool to 55°C(131°F).
Stir in ~ 1/2 cup of yogurt (now down to 2 table spoons).
Leave it alone covered for 6hrs ~ 3 days.  It should curdle in 6hrs.  If you leave it longer the acid develops and whey separates.

The large volume holds the temperature longer. The main bacteria that eats lactose to make yogurt grow very well at high temperature of 55°C. This high temperature suppresses the growth of other bacteria, so you don’t have to worry too much about anything. Just keep everything clean.

Do get a thermometer with an alarm, it really helps. If the milk is too cool you can reheat it to 55°C. My thermometer with an alarm died.  I could not find a cheap one that has cool down alarm.

For starter culture I add two table spoons of the old yogurt. I’ve been able to use this for 6 months now.
When I don’t have a nice previous yogurt culture, I like the brown cow brand plain. The yogurt comes out creamy, smooth, not tart, and almost sweet. It will work with flavored kind, or other brands. Most of the other brands produce yogurt that is too tart or take too long. Don’t use moldy old yogurt. Don’t put sweetener in at this stage it makes things difficult. Sugar invites other contaminating bacteria to grow. These are not toxic but the taste usually suck. Some brands of yogurt (yoplait brand with probiotics for example) use bacteria that do not do well at 55°C and they do not come out well.

You don’t have to use whole milk, but the yogurt is richer this way.