I've been making and drinking kefir for about 6 weeks now and I am in love. Kefir is a fermented milk product that is easy to make at home and very nutritious. There's quite a bit of information about it out there on the Internet, most notably
Dom's Kefir in-site, so I won't repeat it all here. What I will say is that kefir has made a very positive change in my life and I wish I had discovered it sooner. I had been suffering from insomnia and anxiety for several years, but in the past weeks I have noticed these going away. It's very strange. I feel I am at peace and I sleep deeply. I am happy again. I am not sure if I had a nutritional imbalance, or a problem with my internal bacteria/yeast ecosystem, or a combination of the two, or what. And yes, before you make fun of me, kefir is slightly alcoholic, but no, I'm not just drunk! It's got a very small amount of alcohol, less than 1%.
Kefir making starts with some globules called "grains." They are whitish gelatinous blobs that look a bit like cauliflower. I got my grains from a woman on the Internet,
Marilyn Kefirlady. You can get yours from her or you could try craigslist or
this link. Most people will either give them away, or charge you a small "hassle fee" ($15-$20) that covers shipping costs, or makes it worth their time because you are 20 minutes late to meet them, etc.
Once you get your kefir grains, you need to feed them milk. Put them in a glass jar with a quantity of milk. I use about a pint. The experts say to use enough milk so that the milk is fermented in 12-24 hours. You can tell when fermentation has happened because it gets "popcorny." (I can't take credit for the term
popcorny. Credit goes to this guy James that I shared grains with who gave me some tips.) Popcorny means that the grains are floating at the top in a layer of curds and whey and, well, it looks like popcorn. The degree of popcorniness that you let it get to is up to you. Here's two pictures of different degrees of popcorniness. The one on the left is less popcorny than the one on the right.

If you swirl the jar around, the popcorniness goes away temporarily and it looks like this picture.

The time it takes to ferment is a combination of temperature (warmer=faster), amount of grains (more=faster), and amount of milk (less=faster). The more fermented it gets, the more sour it gets. I haven't figured out the right proportions of these variables to make nice, thick kefir yet. I am working on that. Mine usually turns out thin with curds floating in it. It's fine that way, but I would like to see if I can make it thick.
Okay, once it gets fermented what do you do? You strain out the grains and keep the fermented milk. The experts say that you want your strainer to be made of a non-reactive material and there is some controversy on what material to use (plastic versus stainless steel versus bamboo). I use a stainless steel strainer and am happy. These are picture of my grains after I have strained the kefir into a measuring cup. The left one is immediately after straining. You see there are some curds in the strainer. The middle one is after I have jiggled the grains around a bit in the strainer to get some of the curds to go through into the measuring cup. The one on the right is me getting the grains out of the strainer to make more kefir. The important bits here are: 1) my beautiful pink spatula and 2) you should be nice and gentle with the grains -- don't smash them or poke them.

Now, you put the grains into a clean, glass jar and start again. You don't need to rinse the grains or anything. Just leave any curds that are stuck to them where they are. If you notice any sliminess, this is good.
The slimy stuff is a polysaccharide that is made during the fermentation process. It's good for you; don't worry about it (see for example
this article). Then add milk to the glass jar with the grains and leave it to ferment. Take the kefir you just strained and either drink it straight away or put it in a clean glass jar in the refrigerator. The experts say that it will keep for two weeks in the fridge. These are pictures of this step. On the left is the kefir grains after I just put them into a new glass jar before adding the milk. On the right is the kefir I just made sitting in the fridge.

Okay, what do you do with the kefir after you have it? Well, there's lots of things to do with it. I just drink it so far. I either make a smoothie by putting it in the blender with some frozen fruit like strawberries or blueberries, or if I don't feel like washing the blender, I mix the kefir with a little agave syrup (honey would work, too) and some cinnamon. Kefir is, in my opinion, an acquired taste, so give it a chance before you give up on it. Remember how bad coffee tasted when you first tried it and now you can't live without it? This is kind of like that.

Over time, your kefir grains will multiply. They get bigger and then some babies seem to pop off of them. What do you do with the excess grains? You can share them with friends or random people on craigslist. Or, crazy as it sounds, you can eat them. Eating them is reported to have more probiotic benefits than simply drinking the kefir. Yes, I admit I have eaten them and have lived to tell the story. It took me a while to work up the nerve (several weeks!), but I put a bit (approx 1-2 tsp) into the blender when I was making a kefir smoothie and blended it
really well. The result was fantastic. The smoothie had a nice creaminess to it. Dom
reports that he cured his ulcerative colitis by drinking kefir and eating the grains.