Shake, strain, ice-filled highball glass, garnish.
Don's Mix is 2 parts grapefruit juice to 1 part cinnamon syrup.
The posted recipe has been verified with the creator. The recipe in Beach Bum Berry Remixed was incorrectly calling for Donn's Spices #2. This error has propagated to other on-line sources.
Beach Bum Berry Remixed