Here is Katz's yogurt recipe; I think it's the best:
Timeframe: 8-24 hours
Insulated cooler (or sleeping bag/warm blankets--this is what we use!)
Ingredients (for 1 quart):
1 quart/1 liter whole milk (or any kind of milk including soy, coconut, rice, etc.)
1 tablespoon/15 milliliters fresh live-culture plain yogurt for starter culture
1. Preheat the jar and insulated cooler (I put a sleeping bag in the drier to get it warm, but this is not necessary) with hot water so they will not drain heat from the yogurt and it can stay warm to ferment.
2. Heat the milk until bubbles begin to form. If you use a thermometer, heat milk to 180 degrees F. Use gentle heat, and stir frequently, to avoid burning the milk. If does not need to come to a full boil. The heating is not absolutely necessary, but it results in a thicker yogurt.
3. Cool the mild to 110 degrees F, or the point where it feels hot, but it is not hard to keep you (clean!) finger in it. You can speed the cooling process by setting the pot with the hot milk into a bowl or pot of cold water. Don't let the milk get too cool; the yogurt cultures are most active in the above-body-temperature range.
4. Mix starter yogurt into the milk. Use just 1 tablespoon (15 milliliters) per quart. I used to use more starter, assuming that more is better, until I consulted my number one kitchen reference book, The Joy of Cooking (1964 edition), known affectionately as "Joy" in our kitchen. "You may wonder why so little starter is used and think that a little more will produce a better result. It won't. The bacillus, if crowded, gives a sour, watery product. But if the culture has sufficient Lebensraum (German for 'room to live'), it will be rich, mild and creamy." Mix the starter thoroughly into the milk, and pour the mixture into the preheated jar.
5. Cap the jar and place it in the preheated insulated cooler (or sleeping bag). If much space remains in the cooler, fill it will bottles of hot water (not too hot to touch) and/or towels. Close the cooler. Place the cooler in a warm spot where it will not be disturbed. "Yogurt has the added idiosyncrasy that it doesn't care to be jostled while growing," notes Joy.
6. Check the yogurt after 8 to 12 hours. It should have a tangy flavor and some thickness. If it isn't thick (hasn't "yoged"), warm it up by filling the insulated cooler with hot water around the jar of yogurt, adding more starter, and leaving it 4 to 8 hours. You can leave it to ferment longer if you wish. It will become more sour, as more of the milk's lactose is converted into lactic acid. A longer fermentation period can often make yogurt digestible even for lactose-intolerant individuals.
7. Yogurt can store in the refrigerator for weeks, though its flavor will become more sour over time. Save some of you yogurt to use as a starter for the next batch.
****Please don't be intimidated by the length of this recipe! Once you do it the first time, you get it and it takes no time/effort at all. I included this detailed recipe because I want ya'll to have great results the first time and become life-long yogurt makers, ok?!
Happy yogurt making.