You need like two tablespoons of yoghurt per liter of milk. Just cook the milk beforehand and let it cool to about 45°C. Mix in the yoghurt and put it in an oven with the light on for a few hours. Voilà!
Your post triggered really bad feelings in me (not your fault) since my last ex got into a really bad episode and just kinda… Forgot about me. That was fun :D