Fill the stones (there may still be fruit residues) in a large container with a wide opening (e.g. a large mason jar, etc.). Add the sugar and pour the alcohol on top.
Screw the glass on tightly, turn it upside down a couple of times and then back again so that the sugar begins to dissolve and trickles into all the gaps. Let stand in a dark, cool place for 3 - 4 months.
Then strain (best with a measuring cup) and fill into a bottle. Tastes good chilled like exotic fruits and is very popular with guests as an aperitif, digestif or mixed with sparkling wine or prosecco.