Here is our easy no-fail yogurt method:
You will need:
1 gallon (4 litres) milk (use raw, or not ultra high pasteurized milk)
1/4 cup live culture yogurt from your last batch (or from the store)
Clean sterilized mason jars, rings and lids
Large stainless steel pot
Several towels or old blanket
Heat the milk to just under 100F (do not go over 100F, as this kills the living cultures). Heat over low to medium heat to avoid scalding, and stir occasionally. Have your jars ready - washed, dried, and sterilized by pouring boiling water in them. Set them into a heat proof baking pan (I use a large glass cake pan).
When milk has warmed sufficiently, take off heat and gently stir in the yogurt culture. Pour into your clean jars, cap with lids but do not tighten lids too much. Pour boiled water into the pan around the jars, and wrap in old towels. Set in a warm, draft-free place like a kitchen counter. Add more hot water throughout the day to keep the temperature as even as possible. Other people set the jars on a heating pad, or near a wood stove or oven. The milk will still form yogurt if you don't keep heat even, but it will be a nicer thicker yogurt if it's warmed longer...pour off the whey which may have separated, and use this in smoothies, or baking, etc.
Unwrap the next day, after aprox. 24 hours. Add honey or fruit if you like for sweetener. Refrigerate and make sure to reserve a little of this yogurt for your next batch! Enjoy!