TO MAKE YOGURT:

Pour desired quantity of MILK into a clean, wide-mouth glass jar(s).  Any % fat milk can be used; but lower % s will be more tart and less creamy.  I use 2 % or 4 %.

Heat to about 180 degrees F.; I do this in the microwave.  (You can use full power at first, but as the milk warms up, use a lower setting, to keep the milk from boiling over all over the microwave.)

Let the milk cool, still in the microwave, to about 125 degrees.  Then cover jar top with a clean sandwich bag, and put it in a place it can stay warm.  I use a mini-cooler.  Let milk continue to cool, to 90 to 100 degrees.  (Temperatures over 135 will kill the yogurt culture.  Temperatures over 100 will make the culture set too quickly, resulting in a curdled sort of texture.  Moving the jar to your cooler when the milk is 125 degrees warms up the cooler just enough.)

Stir YOGURT (see note below) into the milk, about 1 Tbsp. yogurt per cup of milk.  Re-cover jar top.  Place jar(s) back in the mini-cooler.  Leave undisturbed for about 8 hours.  Jiggling can disturb the culture and keep it from "gelling."  Milk should become the consistency of a light custard.  After chilling in the refrigerator, it will become a little firmer.  You have yogurt!

NOTE:  Yogurt for starter can come from your last batch, or from some purchased at the store.  if purchased, it has to have active yogurt culture as an ingredient.  My favorite for a creamy texture is Brown Cow.

TO MAKE YOGURT CHEESE/GREEK YOGURT:

Strain yogurt (either homemade, or commercial without added gelling agents), using a colander lined with coffee filters/s or food-safe paper towels, over a bowl, in the refrigerator, for 12 to 24 hours.  Discard the clear whey that drains off.  (some say it makes a refreshing drink, but I'm not buying it.)  The resulting yogurt cheese is a wonderful substitute for sour cream to make dip, top potatoes, sweeten for a fresh fruit sauce, etc.

But... beware that it will separate/curdle if cooked!  If you want to try using it in place of sour cream in a cooked sauce, try adding 1-2 Tbsp. of cornstarch per cup of yogurt cheese, thoroughly mixed into the yogurt cheese.