Ingredients: milk, bacterial culture.
How? Heat Milk to simmer (+/- 80 C), let cool to +/- 40 C, add bacteria (from yoghurt and/or pro-biotic tablets), place in oven for 24 hr with oven light on such that temperature is around 40 C (usually half-way across oven from oven light in ours). Enjoy!
Cost is generally one-third or a quarter of buying yoghurt in store.