You may have only seen Boston shakers behind bars but with these steps you can master it at home.
Assemble ingredients in mixing glass, then fill with ice. Invert mixing glass and its contents into shaker tin and angle glass so that one side is flush with tin. Firmly tap base of glass with heel of hand to form tight seal.
Hold base of shaker tin firmly with one hand and base of mixing glass firmly with other hand. Using vigorous back-and-forth motion (ice should hit both ends of shaker), shake cocktail as specified in recipe.
To break seal, position shaker on counter with mixing glass on top and angled toward you. Grip side of shaker tin with one hand and, using heel of other hand, firmly tap rim of tin on opposite side. (You may have to do this more than once.) Remove mixing glass.
Fit Hawthorne strainer onto shaker tin and decant cocktail into chilled serving glass, using conical strainer if directed.