It can be as easy as putting 1/2 cup plain yogurt (organic so you know it's alive) into a quart of milk - then letting it sit in a warm place for 8-12 hours.

I have a yogurt incubator from Tribest and buy sweet yogurt starter from a www.cheesemaking.com (though I am now using the yogurt I make with the starter to make more yogurt at least one generation). It's more scientific this way - and the yogurt starter I'm using makes the most amazing yogurt.

Give it a try - it's fun, and it makes you feel like a hippy :-)