this is so easy it's ridiculous! this method makes rich, creamy yogurt that you'll want to consume in outrageous quantities. when I was a kid, my dad went through a yogurt-making phase, but we never knew to strain it to make this wonderful greek-style yogurt. I happen to think my method's a bit easier (the hubris of a child!). here it is:
1 gallon whole milk (or other, depending on your needs/taste; this is just what I prefer)
1 cup fresh, unadulterated yogurt (when I first started making my own yogurt, I used my favorite brand of greek-style yogurt, but now I just save a cup of each batch I make to innoculate the subsequent batch of yogurt)
heat the gallon of whole milk to 175-185 F (***do as I SAY, not as I do: I find that milk tastes too "cooked" for my taste at 185 F, so I heat to about 150 F without problems***) -- use a food thermometer for this -- DON'T GUESS! heat it slowly, so it won't get scorched on the bottom
fill your sink with ice water and plunge the pot into it to cool the milk to 110 F (again, use the thermometer for accuracy), making sure the water level is appoximately equal to the level of yogurt in the pot
add the cup of yogurt to the cooled milk, whisking thoroughly
cover up the pot and place it where it will maintain its temperature for 6-8 hours (I use my gas oven, off, but with the pilot light on, and it works perfectly)
after the time is up, check the yogurt: it should be thickened and somewhat tart (leave it another hour or two if you want it thicker and/or more tart). it will be NOWHERE near as thick as the final product we're making, though; first we have to drain off about half of the whey. to do that:
sterilize ALL the following equipment, including the cheesecloth (or other fabric used to strain the yogurt): colander, cheesecloth, large glass bowl, large plate
line a metal colander with a large cheesecloth or other fine, smooth fabric (I use a pillowcase that I wash with detergent and bleach each time before use), and place that lined colander in a larger glass bowl. pour the yogurt into this contraption, fold the fabric over loosely over the top of the yogurt, and cover it all with a large plate, then place this in the refrigerator. drain the yogurt for 1-3 hours, until thickened to your preference. SAVE THE WHEY! it has many many uses (my friend Charles says "SAVE THE WHEY!" would make a great bumper sticker).
scrape the thickened yogurt into sterile containers, label with the date and refrigerate. this IS the same as the yummy, thick greek-style yogurt you can buy at the store -- but now, you can control how tart it is (longer fermentation = more tart yogurt), the fat content (obviously, by choosing skim, lowfat, reduced fat or whole milk) and the thickness. it's good stuff, PERFECT for my favorite frozen yogurt recipe (that's next)!