Wisehedgecrone - exact quantities are not strictly necessary, but the principle is 1lb damsons (or sloes), 6 oz sugar and enough gin to top up and cover the fruit. (cheap supermarket gin, own brand, is ideal).
Normally you would prick the skins, traditionally with a darning needle which is, tbh, arduous, but a fork with sharp tines is fine. If, however, the fruit has been frozen, the skins may have split and this process won't be necessary. But the puncturing of the skins allows the full flavour of the fruit to be extracted, and the colour of the skins to colour the gin.
Put the prepared fruit and the sugar in a wide-necked jar and top up with gin to cover the fruit. keep in a cool dark place, and gently shake the jar each day for a couple of weeks to ensure all the sugar is dissolved. Leave for three months (longer if you can manage it!) and strain into clean bottles. (I use wine vinegar bottles with the labels soaked off, of course - they're a good size and, if you're feeling generous, you can give them as Christmas presents).