Don't worry about using an expensive vermouth for this; an inexpensive one will work just as well.
Directions
Shake or stir with ice, and serve with ice.
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, 1/6 French vermouth, 2/6 Campari.