Wash and sterilise a large (at least 1 litre) jar and lid.
Remove all the stems and leaves from the cherries, but don't bother to pit them, and wash them under a tap.
Put the washed fruit and sugar into the jar, pour in the gin until the cherries are well covered - or to the top of the jar if you want.
Put the lid on tightly and shake.
Store on a cool, dark shelf and shake daily until all the sugar is dissolved.
Leave for a minimum of 3 months before drinking, then strain the liqueur, discarding the fruit.