Start the day before, and rinse the damsons, and pick over to remove any leaves or stems. Pat dry and put into a freezer bag, and freeze overnight.
The next day, remove the bag from the freezer and bash the damsons a couple of times with a rolling pan (skip this step if using sloes). Tip into a 2-litre jar.
Add the sugar and gin, and put the lid on. Shake well. Every day for one week, shake the jar well until all the sugar has dissolved. Then leave the jar in a cool, dark place for a couple of months.
When you're ready to bottle this off, you'll need a coffee filter paper and a funnel. Make the paper into a cone and insert into the funnel. Strain the gin into clean, dry bottles, then seal and label.
Your gin will be ready to drink now but will continue to improve and mature over time, so you could start this year for Christmas next year.