Here's how you make a Canton Campari. You go to your local supermikado and buy some green fruit, which you believe to be very large limes (though given the small size of the other limes you have been buying you have some strong reservations).
On arriving home you discover the green things are actually mandarins. You juice one. And because your juicer is yet to arrive, you actually just smoosh up the mandarin with your hand into a bowl. How elegant.
Juice a quarter ruby re grapefruit (see method outlined above).
Add ice to a glass. Add a good teaspoon of castor sugar to the ice (really actually a tablespoon).
Put the green mandarin and ruby grapefruit juice into the glass. Add a big dash of campari. Add about a half of that sized dash of gin and stir. Drink while smugly looking at your incredible view.