Rinse and pick over the sloes, then pat dry in a clean tea towel. Using a stainless-steel fork or cocktail stick, prick the sloes and tip into a 2-litre Kilner jar, or divide between 2 smaller jars.
Add the sugar and gin, and seal the top. Shake well. Each day for a week, give the jar a good shake, then put it in a cool, dark place and leave for 2-3 months.
Line a plastic sieve with a square of muslin, set over a bowl, and strain the sloe gin through it. Decant into clean, dry bottles, seal and label. The sloe gin is now ready to drink but will improve and mature over time, so if possible make it one year to drink the next.