Right, many recipes in this thread are fancy. No need to get in such a ferment about it (pun intended). May be needed for colder northern climes, but I have my doubts. (E.g. people were making yogurt in similarly cold Central Asia for centuries, with out such data structures and algorithms, er, containers and instructions.) In India, mothers and housewives (and nowadays househusbands) routinely make curd (Indian English name for yogurt, dahi in Hindi), just by warming (c)old milk (or cooling just-boiled new milk down some), bunging some old curd in (e.g. 1 to 2 tbsp per liter milk) putting it in a slightly cooler or warmer part of the house, depending on the season), and forgetting about it for 8 to 24 hours. That's it. Dead simple. If not come out well, tweak a parameter or two, retry. But mostly it goes smoothly (pun not intended but retained).
Even a fridge is not needed. Put the milk+yogurt mix container in a clay pot placed in a tray of water, and it will turn into yogurt.
Anything fancier on top is just to reduce the amount of work needed or have longer fridge life.