Rinse and pick over the sloes to remove any leaves and stalks, then pat dry. Prick the sloes using a stainless steel fork or cocktail stick, and tip into a 2-litre jar, or divide between two smaller jars.
Add the sugar and vodka, then seal the jar and 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 (or use a coffee filter in a cone for a really refined vodka) and strain the sloe vodka through it. Decant into clean, dry bottles, then seal and label. The sloe vodka is now ready to drink but it'll improve and mature over time – so, if possible, make it one year to drink the next.