The ratio of the two things is not fixed. but 5 parts Bourbon, 1 part Sweet Vermouth and a splash of Bitters

Directions

Put your Martini Glass in the freezer for an hour

Pour 1 part cold (from the fridge) Vermouth in the glass and add 5 parts Bourbon from a cocktail shaker with ice.

The classic garnish is a Maraschino Cherry, I like a twist of orange zest