I started this because I was doing a trash audit (at the time plastic yogurt cups weren't recyclable -- they still aren't, many places) and yogurt containers were a big contributor to our trash. (Even though we were purchasing larger containers.)
Our "recipe":
1/2 gallon milk (we get it delivered, so it's not super-cheap, but it is grass/silage fed and local, which I care about)
1 small container plain brown cow yogurt (you won't use it all, and it doesn't need to be this brand -- this gives me a thickness that I like, but be sure to choose something with "live cultures")
Also,
two wide-mouth quart glass jars,
one travel-sized cooler (the size that holds two six-packs of cans is what we have),
one heating pad (ours is filled with rice, and gets heated in the microwave),
a couple of dishtowels.
Glass measuring cup, or something that'll hold about 1 cup with room to stir.
Warm the milk until it's just starting to froth/expand. Then turn it off. Let cool to around 110 degrees F.
Meanwhile, take two clean wide-mouth quart jars (ours used to hold .. sauerkraut? maybe?). Rinse them out with boiling water, including the lids. Put upside down on a clean dishtowel to dry out a bit.
When the milk has cooled to 110, spoon a little less then half the yogurt into a glass measuring cup (or anything else: I like this because it has a spout and is easy to clean). Add several spoonfuls of the warmed/cooled milk, and stir until the mixture is smooth. Dump it back into the pot, and stir.
Begin heating the hot pad in the microwave, if that's the kind you're using.
Pour the milk mixture carefully into the clean jars (I do the pouring in the sink, because that way stray spills are no big deal). Wipe off the jar lips, if you need to, put on the lids, and put them in the cooler. Wrap with towels and add the hot pack. (The idea is to keep things warm for a bunch of hours). Close the cooler. Go to bed, or away for 7-10 hours.
When you get up/return, take out a bottle and have a look by tipping it slightly. The top layer should be solid-ish. Put in the fridge -- it'll continue to set up while it cools.
If you want it "fruit", add a spoonful of jam (or honey, or frozen fruit) when you eat it. We have ours with granola (which I also make, and is only slightly sweetened) and fresh fruit or applesauce (in the winter). Or we use it in savory dishes.