It’s not as simple as it sounds. Ideally everything including the glass should be cold. You can use a tumbler, a highball glass or one of those fishbowl things that are popular in Spain. Fill the glass with ice; the more ice, the slower it melts meaning less dilution. Add one part gin to two parts tonic (or one to one if you’re feeling frisky). Make sure your tonic is full of fizz, ie not from that two litre bottle of Schweppes that you opened last month. Add a segment of fresh lemon and give it a good squeeze. Other popular garnishes include lime, cucumber or rosemary. Give it a good stir and serve with some ready salted Hula Hoops.