If you are preparing just one or two, you can just stir together right in your tumbler or old-fashioned glass.
Variant 1: top up with soda water and add a slice of lemon.
Variant 2: add a splash of French vermouth as well as the Italian. The proportions would be roughly 3/6 Italian vermouth, ⅙ French vermouth, 2/6 Campari.
Notes
Don't worry about using an expensive vermouth for this; an inexpensive one will work just as well.