It turns out making yogurt is really easy. In fact, the bacteria do all the work. You just have to give them a nice place to multiply, and pretty soon you get to eat them along with the delicious product they've made (transformed?) for you. I was still nervous, though, when I tried it out for the first time. I've ended up with decidedly mediocre results from projects that could be described in similar terms. The beer I invested over a hundred dollars in brewing was pretty pathetic. I was afraid to even try my preserved lemons, and abandoned them when I moved so that they're now a safe half-dozen states away. I've turned out a few inedible loaves of bread. But no matter! Making yogurt, as I said, is really easy. I have a 100% success rate.
These were the sources I used, the high and the low: a NYT article by Harold McGee (a culinary guiding light of mine) and a wikiHow entry. Both are highly recommended reading.
Special equipment: a large microwave-safe bowl (I used Pyrex or something similar). A thermometer with range of at least 100 F - 200 F. Either an oven or some kitchen towels.
I microwaved about a quart and a half of milk for around 8-10 minutes, until it reached around 190 degrees F. Using the microwave once more, I kept the milk between 180 F and 190 F for the next 15 minutes, then cooled it in a water bath to 115 F. The cooling happened surprisingly quickly - less than 10 minutes. I then mixed a few tablespoons of store-bought yogurt (a kind with a bunch of active bacterial cultures) with a similar amount of warm milk, and stirred the milk-yogurt blend into the warm milk. After that the idea is to keep the milk warm - around 100 F - for four or more hours. Luckily, the bacterial growth seems to be exothermic, so after failing to wrap the measuring bowl I had my milk in with kitchen towels, I briefly warmed up my oven until it felt around 100 F, then turned its internal light on and stuck the bowl in there. Harold McGee recommends letting the milk (or soon-to-be yogurt) sit for 4 hours; the wikiHow article wants you to start with seven hours. I didn't time my batch right, so it sat in my oven for almost 12 hours, and it still came out fine: pleasantly tangy and a bit sweet. After sitting, the yogurt needs to be mixed and refrigerated to firm up. The next night, I set some in a strainer lined with a paper coffee filter and put the strainer over a bowl in the fridge overnight, which gave the yogurt a super-thick and creamy Greek-style texture.