The spritz has packed up for the year, the tonic has lost its nerve, and the drink that steps into the gap is redder, bitterer and far more grown up. Negroni Week runs from the 21st to the 27th of September, which gives you a fortnight to get your equal parts in order. Three ingredients, one glass, no shaking and nowhere to hide. The Negroni is the easiest great cocktail in the world to make badly, and one of the most satisfying to finally make well.

There is a reason the Negroni sits near the very top of Drinks International's annual ranking of the world's best-selling classic cocktails, a list it has topped outright in recent years, in the same rarefied company as the Martini it usually outsells. It is a drink of almost aggressive simplicity: gin, Campari, sweet vermouth, in equal measure. No fresh juice, no sugar syrup, no faff. And yet it manages to taste like something a chemist and a poet built together, all bitter orange and dark herbs and a warmth that lands somewhere behind the ribs.

Legend hands the invention to Count Camillo Negroni, who in Florence around 1919 asked a barman to stiffen his Americano by swapping the soda water for gin. Whether or not the Count was quite as advertised, the logic was sound, and a hundred-odd years later the drink he ordered has become the house aperitivo of half the restaurants you love. It is the great Italian before-dinner ritual in a single glass: bitter enough to sharpen the appetite, strong enough to mark the end of the working day. Here is how to make one that earns its place.

The formula

Learn one ratio and you will never be lost: one part gin, one part Campari, one part sweet vermouth. That is the whole recipe. Twenty-five millilitres of each if you are measuring, or one confident glug of all three if you are not. Build it straight into a rocks glass over a big piece of ice, stir until the outside frosts, and finish with a twist of orange. Under a minute, start to finish, and no cocktail shaker anywhere in sight.

The equal-parts version is the classic, and it is the one to master before you start editing. Once you know how the standard tastes, you can begin to nudge: a little less Campari for a gentler drink, a little more gin for a drier, stronger one, a heavier hand with the vermouth if you like it plush. But learn the one-to-one-to-one first. It is balanced the way it is for a reason, and every good variation is a departure from it rather than a replacement.

The gin

The gin is the drink's backbone, and a classic London Dry is the safe, correct answer. You want juniper, citrus and a bit of pine-forest structure to stand up to the Campari, not a soft, floral, cucumber-and-rose number that gets flattened in the first round. Sipsmith London Dry Gin is exactly the sort of thing: bold, juniper-forward, made for stirring into something bitter. If you want to trade up, Hepple Gin brings a greener, more aromatic lift, all Douglas fir and blackcurrant leaf off the Northumberland moors, and it gives the finished Negroni a fragrance that rides right over the top of the bitterness.

The bitter heart

Campari is the drink's whole personality, and there is really no substituting it if you want the Negroni that people mean when they say Negroni. That vivid, unmistakable red is bittersweet and faintly medicinal in the best possible way, all bitter orange and rhubarb and a long, dry finish. Campari Original was first mixed by Gaspare Campari behind his Milan bar in 1860, and the recipe has stayed a closely guarded secret ever since. It is the ingredient that makes the drink feel adult: the one that puts a sceptic off on the first sip and hooks them for life by the third.

The sweet vermouth

Here is where most home Negronis quietly go wrong. The third ingredient is not an afterthought; it is the plush red cushion the whole drink sits on. A rich Italian sweet vermouth is what you are after, and Carpano Antica Formula is the gold standard: dark, spiced, almost chocolatey, with a vanilla depth that rounds off Campari's sharp corners and makes the finished drink taste expensive. Antonio Benedetto Carpano more or less invented vermouth in Turin in 1786, so the house can be trusted to know what it is doing. If you want something lighter on its feet, Dolin Rouge from Chambéry is a gentler, more floral take that makes a prettier, less brooding Negroni.

Whichever you choose, remember that vermouth is fortified wine, and it tires quickly once opened. Keep the bottle in the fridge and use it while it is fresh. A flat, oxidised vermouth is the single most common reason a home Negroni tastes thin and sad, and it is also the easiest fault in the whole drink to fix.

Stir, then the orange

A Negroni is stirred, never shaken. Shaking would cloud it and beat air through a drink that is meant to be clear, glossy and still. Fill the glass with as much ice as it will take, ideally one large cube rather than a handful of small ones that melt too fast, pour your three equal parts over the top, and stir gently for twenty or thirty seconds until everything is properly cold and just lightly diluted. That small amount of water is not a flaw. It is what softens the edges and pulls the flavours open.

Then the orange, which matters far more than it looks. Pare a wide strip of peel, twist it hard over the surface so the oils spray across the top of the drink, run it once around the rim and drop it in. That little burst of citrus is the finishing note the whole thing has been building toward. A wheel of orange works too, and looks handsome in the glass, but the twist is where the aroma lives.

When you fancy a change

Once the classic is second nature, the variations open up like a hand of cards. Swap the gin for bourbon and you have a Boulevardier, autumnal and warming and arguably the better drink come October. Build it with a lighter, more floral gin and a blanc vermouth and you drift toward a White Negroni. Lengthen it with soda and a splash of prosecco in place of the gin and you have a Negroni Sbagliato, the mistaken Negroni that had its viral moment a couple of years back and never quite left. And if you want the drink with none of the labour at all, a bottled option like the Hepple Juniper Aged Negroni does the mixing for you and lives happily in the fridge until Friday.

But start with the original. Pour your equal parts, stir like you mean it, twist the orange over the top and sit down with it somewhere the light is going early. Made properly, a Negroni is not a drink you carry around a party; it is a drink you stop for, the full stop at the end of the day. There is no better fortnight to learn it than this one. Happy Negroni Week.