Rinse and pick over the damsons to remove any leaves and stalks, then pat dry, tip into a freezer bag and freeze overnight. The next day, bash the bag of damsons a couple of times with a rolling pin, then tip the lot into a 2-litre jar, or divide between two smaller jars.
Pour in the sugar and vodka, and put the lid on. Shake well. Each day for a week, give the jar a good shake until all the sugar has dissolved, then put it 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 damson gin through it. Decant into clean, dry bottles, then seal and label. The vodka is now ready to drink, but will improve and mature over time – it will keep for over a year, if you can wait that long.