You'll need:
3oz vodka or gin
1/2 oz dry vermouth
1/2 oz or more of olive juice
Cocktail olives for a garnish
In a cocktail shaker with ice, layer the vodka, vermouth and olive juice. Shake to combine. Strain into a chilled cocktail glass and garnish with the olives, either on a decorative pick, or by floating them in the glass. I like my martini with one ice cube (as above), but it is usually served straight up.