First of all, get all of the major components cold. This essentially means that your Gin should live in the freezer forever, and the to tonic must find a home in the coldest part of the fridge. In addition, store your limes somewhere cold, and chill a lowball glass in the icebox about 10 minutes before you need to use it
When you're ready for that refreshing drink, remove the glass from the freezer and drop in a 2 large chunks of the purest ice you can muster
Slowly pour over 40ml of Tanqueray Gin (50ml if using something around the 40% mark like Beefeater or Gordon's). Pouring too quickly may cause the ice to crack, and we don't want that
Next, slowly add 100ml of the finest Tonic Water you can find. Many bartenders will use a barspoon for this. However, I prefer to tilt the glass to 45° and gently pour it down the edge. Just dumping it into the glass will result in a loss in carbonation
After that, squeeze in 10ml of fresh Lime juice and give everything a gentle stir
Finally, take a small sprig of fresh rosemary and give it a little squeeze to help release its aroma, as well as a slice of lemon zest, scratching the skin as you go for the same reason. Drop them both into the drink and give it one final stir
Sit back and enjoy the best Gin and Tonic you are ever likely to find