Freeze overnight. 1/2 fill a large kilner jar. Fill with vodka. Agitate regularly. As long as possible but at least 6 months. When you can bear it no more, remove the damsons (keep them for a punchy pudding) and add sugar to taste.
With all these drinks, the trick is to add the sugar after you have finished steeping the fruits. The flavour comes out better in the absence of sugar and you can keep adding sugar in small increments until you get the levels right. Hard not to get pi$$ed while getting the levels right but that is half the fun. Don't use the cheapest vodka, don't use premium either.
Hedge