1. Boil milk in a pot (3-4 liters). Forget it's there. Turn it off when someone notices it boiled.
2. Wait till it cools to ~40c. Try to put a finger in the milk. If it's cold enough so you don't have to take your finger out, it's about ready.
3. Dump a cup of probiotic yougurt inside and stir.
4. Cover with the lid. In the winter wrap it with some blanket to keep it warm (probably not really necessary)
5. Wait for 24 hours.
6. It's ready. Put the whole thing in the fridge.
Somehow this works every time... It's seems really hard to ruin the process, and not for lack of trying...