Here's the recipe I use for damson gin and you can adapt it for sloe gin, perhaps adding a smidge more sugar.
You will need:
700 ml vodka or gin
440 (ish) g damsons (stab with fork)
350g caster sugar
For sloe gin you need to prick the sloes allover with a needle. I used a safety pin and because the sloes are small you can leave this in a bottle. A delicious treat. Pour into interesting bottles and give as Christmas presents. Chin chin!