Add ice cubes to your glass. I like to add the ice first so I have a good idea of how much space I have left in the glass. Then fill about 1/3 of the glass with Campari. Then fill the rest of the glass with club soda. You can adjust the proportions to suit your taste, but a 1/3 campari and 2/3 club soda is a general rule of thumb I like to follow.