3 measures black currant drink mix
1.5 measures citrus-based liquer (Grand Marnier, Famous Grouse - even though they are really different, they both work equally well)
Shake and strain over rocks into a Saloon Whiskey glass - preferably from before 1900 because newer ones just don't have the same charm.
Fill with soda or ice-water to the top
Garnish with orange peel