0.5 liter cream
1 big tablespoon of sour cream of the shops
Put these mixed together in a closed glass jar and leave for one to two days at room temparature. The substance should start to thicken after one day, if it's not thick enough, leave it for some more time. Check back regularly until the desired thickness is achieved.
Put in the fridge.
Yes, it's that easy.
Instead of the readymade sour cream you can also add butter milk to the cream, but this gives a slightly different taste, I don't really like personally.