[bernawil]

Here's my lazy technique.

Get the biggest thermos flask you have. Get as much milk as it fits.

Get any yogurt. So far literally anything has worked: flavored, with added sugar or even the cheap versions with lots of gelatin.

Heat the milk around until it boils. Add sugar until it's slightly sweet but not too much (this will feed the culture. It won't be sweet after). Let it chill until 40c or slightly above body temperature.

Now mix in the yogurt, you need like 6 teaspoons for a liter milk, but just pour the container you have for good measure.

Pour the mix in the thermos flask and leave for aprox 4 hours, or until the whey separates. After that, the longer you wait the more acidic the yogurt gets.

Now time to strain. I put a sheet of coffee filter paper on the bottom of a pasta colander and another container beneath to collect the liquid. Pour the yogurt there, leave overnight in the fridge and you get thick Greek yogurt.