Add the grapefruit juice to a cocktail shaker. If you're using freshly squeezed juice, pour it through a fine mesh strainer to get rid of the pulp and seeds.
Add the cinnamon syrup to the cocktail shaker.
Fill the shaker with ice and shake for only about 10 seconds. This will chill the mixture.
Strain the mixture into your cocktail serving glass.
Fill the glass with the grapefruit seltzer.
Add the fresh grapefruit garnish, and serve.
Notes
To make your own cinnamon syrup, try my Cinnamon Syrup recipe (either the regular or keto-friendly version).
To purchase cinnamon syrup, try out DaVinci cinnamon syrup or any available at your local grocery store.
You can substitute in maple syrup if you don't like cinnamon.