The Gin & Tonic is, alongside Pimms, the most English of summer drinks. Sharp and refreshing and (ideally) brutally cold, the combination of gin and bitter tonic is pure magic. The Spanish also love their gin & tonics, often selling various different tonics and gins to match, and preferring to drink theirs from what look like massive wine glasses.
A lot of people have a vague idea that the gin & tonic goes back a long way, but you might not realise how important it is to British history. Not to tread the same ground twice (you can read more detail on gin here), but gin is pretty tied up in the last few hundred years of English history, and especially closely tied to the Navy. As a result, if you were a British naval officer, or otherwise involved in the truly unpleasant business of colonialism, it was probably the spirit that you had to hand at the end of the day. In fact, a lot of the 'botanicals' in gin – juniper, orris root, angelica, grains of paradise, citrus and so on – would have been brought back from the colonies to make various gins.
Tonic was also very involved in the building of the British Empire, especially in warmer, wetter regions where malaria was common. That's because the ingredient that makes tonic bitter, quinine, acts as a good defense against the symptoms of malaria (it's still used today – not as an anti-malarial, but for the treatment of cramping in sleep). Quinine is also mad sensitive to UV light, which is why your G&T glows a little bit while you dance to Ignition(Remix) in Infernos and cry inside.
Anyway, the first commercial tonic water was produced in 1858. It would have been pretty much a mixture of quinine and soda. The members of the East India Company took to drinking this with the addition of lime, sugar and gin in order to make their medicine more enjoyable, and a classic drink was born.
Modern tonic contains far less quinine, and is made bitter through other ingredients, but it's still great. Recently, more 'artisanal' tonics have appeared, using higher quality ingredients and more natural quinine, although in most countries the amount of quinine is very limited as it's pharmaceutically active. It is possible to buy your own cinchona bark and extract the quinine yourself if you're willing to sign a worrying disclaimer (I get it from Bristol Botanicals), and from that to make your own tonic water (or tonic syrup you can add soda to). I make mine with cinchona extract, orange blossom water, citric acid and sugar. It's pretty amazing (he didn't even attempt to humblebrag).
To make the classic gin and tonic you'll need:
Presuming you've still got your gin left over from Christmas (ha), you just need to grab some tonic. Personally, I can't drink sugar-free tonic, but that's always there. Otherwise, grab some Fevertree or Fentiman's tonic if you're feeling wealthy, or Schweppes or even a supermarket own-brand bottle. If you've got gin at less than 40%, you can use that in your G&Ts and save the strong stuff for martinis.
You're also going to want to grab plenty of ice, some limes and maybe a cucumber.
How to make
The most common mistake with the G&T is to use too much tonic. While drinks like a JD & Coke or Jameson & Ginger Ale hold up to a bit more dilution, the gin and tonic really needs to be pretty punchy with plenty of ice. We're aiming for a ration of anywhere between 1:3 gin to tonic, all the way to 1:1.
Don't be afraid to use a pint glass, and make sure you've got plenty of lime ready. For a good squeeze, you want to trim the ends of the lime, then cut into 6 wedges.
Fill your glass to the brim with ice, and add your gin. Then take a wedge (or two) of lime, and squeeze them into the drink. Run the wedge of lime around the rim of the glass too, to give it a little extra.
Finally, top up with tonic, stir, and drink immediately. Remember, tonic is weirdly effervescent, so pour gently and drink quick enough to make sure it stays fizzy (though, y'know, responsibly, of course).