There is TONS of amazing information here: http://users.sa.chariot.net.au/~dna/Makekefir.html . I always try to use raw milk, but if none is available you can follow the same instructions using pasteurized milk (no heating necessary).
My basic technique:
1/2 gallon of raw milk
1/2 cup milk kefir grains (or use yogourmet kefir starter; follow directions on the package if using starter)
If you're using the grains, mix them with the milk in a large mason jar. Let sit, loosely covered, at room temperature for 24 hours. Stir gently every so often, if you want.
After 24 hours, using clean hands or a spoon, remove the kefir grains, leaving some of their polysaccharide "coating" behind (which looks a lot like eggwhite, and is very tangy).
Use most of the grains to start your next batch of kefir; eat the rest, or share them with friends! (You can keep the strained kefir at room temperature for an additional 24 hours, for a second fermentation before transferring to the fridge.)
When you're not using your grains, you can store them in the fridge in a small jar, covered with fresh milk.
To make smaller batches, use less milk and proportionally less grains.