A Tom Collins cocktail requires gin, lemon juice, club soda, water and sugar. Place ice in a highball or Collins glass until it is one-third to one-half full. Cut a lemon in half and save one half for the garnish. Squeeze the other half of the lemon into a glass to use for the drink mix.
Mix equal amounts of water and sugar to create 3/4 ounce of syrup. Combine the syrup with 1 1/2 ounces of gin and 3/4 ounce of lemon juice in a shaker cup. Fill the shaker with ice, and secure the lid. Shake for 15 to 20 seconds or until the outside of the shaker begins to feel cold. Strain the mixture into the glass without the ice. Fill the remainder of the glass with club soda. Cut the lemon half into wedges or wheels, and place one on the rim of each drink glass as a garnish. Add a maraschino cherry as a second garnish, if desired.
Substituting another liquor for gin creates one of several different Collins cocktail variations, such as a Brandy Collins, Whiskey Collins or Rum Collins. A Charlie Collins consists of Jamaican rum, lime juice and a small amount of Angostura bitters.