Prick the tough skin of the sloes all over with a clean needle and put in a large sterilised jar.
Pour in the sugar and the gin, seal tightly and shake well.
Store in a cool, dark cupboard and shake every other day for a week. Then shake once a week for at least two months.
Strain the sloe gin through muslin into a sterilised bottle.
You can make blackberry brandy in the same way, substituting blackberries for the sloes and brandy for the gin. Blackberries do not need pricking.