Sloes are the fruit of the Blackthorn bush, which are found abundantly in hedgerows in the autumn months. They look a little bit like a cross between a blueberry and a small grape, but are quite bitter and tart if eaten raw.
Many traditional sloe pickers will tell you to wait until after the first frosts to pick your sloes, but I tend to pick them when I come across them and freeze them once they have been washed. This helps break the skins and makes infusion your more potent.
The best time to make your gin is in the autumn so that it has a few months to infuse before Christmas. Alternatively, you can make batches on an annual basis and drink year-old sloe gin in the winter. Maturing the infusion improves the flavour, so patience is definitely a virtue.
If you are not keen on gin you can use the same method to infuse vodka or any other spirit you fancy experimenting with.