Pour the gin and cranberry juice into a glass and fill the glass about 3/4 full with ice cubes. Squeeze a lemon wedge over it, fill up with well chilled tonic water, add the lemon wedge to the glass and garnish with lemon zest.
If you like, garnished with a sugar rim: Put some cranberry juice on a small flat plate and sugar on a second plate. Before filling, dip the edge of the glass in the cranberry juice, then in the sugar.
Lemon peel cut in a spiral shape is also suitable as a decoration.
Note: You don`t need a particularly fine gin for this long drink, as cranberry juice and a large amount of tonic water dominate the taste.