Fill a glass with a capacity of at least one liter up to halfway with plum stones and pour 1 bottle of Doppelkorn (0.7l) over it. Close the glass and put it on the windowsill for 1 year. Shake now and then.
Strain through a cheesecloth after a year. You can add 50-100 g of sugar, depending on your taste. It tastes best not so sweet, so almost without sugar.