Start by taking a cocktail glass (a martini glass will work well for this purpose, although you can also use a rocks or bucket glass), and place it in the refrigerator. This will ensure that the glass is frosted and fully chilled by the time you are done preparing your drink.
Now, while the glass is chilling, start preparing the drink. Take a cocktail shaker and fill about a quarter of it with ice cubes or cracked ice. Once it is loaded with ice, you can start pouring in the ingredients for the Tom Johnstone.
Add all the ingredients into the cocktail shaker one by one – start with the 1 ½ ounces of scotch whiskey, and follow it up with ½ ounce lime juice, ½ ounce Italian vermouth, and ½ ounce Cointreau.
Once all the ingredients have been added into the cocktail shaker, proceed to screw on the cap of the shaker tightly, and then shake until all the ingredients are well blended, and the exterior of the cocktail shaker begins to feel cool.
When you are done with the shaking, take your chilled serving glass out of the refrigerator, hold a strainer over it, and pour in the prepared drink. Your Tom Johnstone cocktail is now ready – garnish with a wedge of fresh lime or lemon, or with twists of lime peel, and serve chilled.