Put a martini glass in the fridge to chill for 10 mins. Pour the gin and vermouth into a mixing glass or jug filled with ice cubes. Stir with a mixing spoon until the outside of the glass is very cold, then taste to check the dilution. If needed, continue to stir until the martini is diluted and chilled to your liking.

Strain the martini into the chilled glass and serve.