The G&T is one of the only drinks in the canon where the difference between bad and brilliant is measured in millimetres. The same gin, the same tonic, the same lime. And yet a properly built one will taste like a country garden in midsummer, while a careless one will taste like a service station forecourt. Most of the difference, it turns out, lives in the details no one bothers with.

There are essentially five things to get right. They are not difficult. They are not, for the most part, expensive. But each one matters more than people realise. And each one is the reason the G&T at a great hotel bar tastes wholly different from the one you've just made yourself at home.

The gin

Begin here, because everything else cooperates with what you choose. A London Dry (juniper-led, dry, properly proofed) is the safest brilliant choice. Tanqueray, Beefeater, Sipsmith for the keen home bartender. The juniper does the structural work; everything else is decoration.

If you want the drink to taste of summer, reach for a contemporary botanical-led gin. Citrus peel, coriander, a dash of grapefruit. Hendrick's, with its cucumber and rose, is built for this. So is Seventy One Gin, the Mayfair-made London Dry with a quietly assertive juniper backbone and just enough citrus to keep it cheerful. Pour 50ml of it into a balloon glass with everything that follows and you'll understand the fuss.

Whatever you choose, pour with conviction. The British tendency to underpour gin is, frankly, a national embarrassment. 50ml is the right measure. Less than that and the tonic flattens it; more than that and the drink starts to lecture.

The tonic

The tonic question is the one most home bartenders never even consider. Most supermarket tonic is too sweet, too aggressive, too synthetic. Built around saccharine rather than quinine. A great gin paired with cheap tonic is a romance ruined by the wedding planner.

Use a premium brand. Fever-Tree is the supermarket bottle to reach for; Double Dutch and Le Tribute are worth seeking out if you care more. A Mediterranean tonic, softer, with a citrus edge, pairs beautifully with botanical gins. A classic Indian tonic, with its more pronounced quinine bite, is the right partner for a juniper-forward London Dry.

One small bottle per drink. Always. A half-drunk litre bottle in the fridge is flat by tomorrow and tastes like apology by Friday.

The ice

Use too much. More than you think. The single most common mistake at home is adding three or four cubes and assuming it's enough. What happens is the ice melts almost immediately and dilutes the drink within minutes. Fill the glass. Properly fill it. To the brim.

Use big, dense cubes if you have them. Failing that, multiple smaller ones. The ice is structural. It is what gives the drink its bracing, almost-painful coldness, the thing that makes the first sip taste like a held breath.

The garnish

Lime is the default, and the default is, mostly, fine. But the right garnish should have a relationship with the gin. A juniper-forward London Dry is happiest with lime and perhaps a sprig of rosemary. A citrus-led gin will sing alongside grapefruit peel: a single, well-pared strip, twisted over the glass to release the oils, then dropped in.

A botanical gin (Hendrick's, especially) loves cucumber: three thin ribbons, peeled with a vegetable peeler, draped down the inside of the glass. A Spanish-style gin tonic (the kind made famous in Barcelona) leans into pink peppercorns, juniper berries, a wedge of orange.

Whatever you use, it should be fresh, generous, and not, under any circumstances, a tired wedge of lime that's been sitting on a bar all afternoon. Citrus oxidises. Tired garnish makes a tired drink.

The glass

Use a balloon. The Spanish copa de balón has become standard for a reason: the wide bowl traps the aromatics from the gin and the garnish, so that every sip is preceded by a noseful of what's about to happen. A standard highball will work. A small wine glass will, in a pinch. A flute will not.

Chill the glass beforehand if you can. Five minutes in the freezer is enough. A frosted glass keeps the drink cold longer and is, frankly, more attractive to drink from.

The build

Fill the glass with ice. Pour 50ml of gin over the ice. Pour the tonic slowly, down the side of the glass, against a bar spoon if you have one, to preserve the bubbles. Garnish. Stir once, gently. Don't agitate it; you've worked too hard for that.

Total assembly time: under thirty seconds. The whole point of the G&T is that it should be impossible to overthink. Get the ingredients right, build it without ceremony, and let it do its work.

Drink it outside, if you can. A G&T in a cold conservatory, on a terrace at six o'clock, in a deck chair before lunch, in a bath at the end of a difficult Tuesday: all of these are correct. A G&T at a kitchen table, with the dog at your feet and nothing particular to do, is also correct. The drink is unfussy. The drink rewards effort but does not require ceremony. That is what makes it perfect.