Fill a glass with ice, and pour the Campari over top.
Top with club soda, and stir.
Garnish with a slice of orange.
Notes
The ratio to follow for this cocktail is 1 part Campari, 3 parts club soda. For this particular recipe, I used a tall glass, with 2 oz Campari and 6 oz club soda. You can adjust using this ratio to the size of your glass.
I like using club soda for this cocktail, but you could also use soda water, seltzer, or mineral water.
I recommend using a shot glass to measure out the Campari and club soda in order to get precise measurements.
To garnish with the orange, you can simply drop an orange slice into the cocktail, or place it on the rim of the glass.