Squeeze the orange into a cocktail shaker and add the cranberries and simple syrup. Muddle with a cocktail muddler until the cranberries are well juiced.
Add the whiskey and ice and shake vigorously for 30 seconds. Strain into a chilled glass and garnish with the cranberry and orange slice.
Video
Notes
You may need to strain twice depending on how fine your cocktail strainer is.