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 two smaller jars.
Add the sugar and gin, then seal the jar. Shake well. Once a day for seven days, give the jar a good shake. Store the jar in a cool, dark place and leave for two to three 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, then seal and label. The sloe gin is now ready to drink but it'll improve and mature over time – so, if possible, make it one year to drink the next.