Chill a martini glass (optional: chill a mixing glass too).
Add ice to a mixing glass.
Pour in gin (or vodka) — typically about 2 1/2 to 3 oz.
Pour in dry vermouth — typically about 1/2 oz (for a “dirty” martini, keep it minimal if desired).
Add olive brine — typically about 1/4 to 1/2 oz, to taste.
Stir with a bar spoon until well chilled (about 20–30 seconds).
Strain into the chilled martini glass.
Garnish with an olive (or a lemon twist, optional).