Place the fresh cranberries into a muddler or a sturdy glass and gently crush them, releasing their bright red juice and aroma, until they are slightly broken but not completely pulped.
Add a splash of orange liqueur and a teaspoon of cranberry syrup to the muddled cranberries. Gently stir with a spoon to combine and infuse the flavors.
Fill each champagne flute halfway with the cranberry mixture, allowing the vibrant red juice and bits of cranberry to fill the glass with color and texture.
Slowly pour chilled champagne or sparkling wine over the cranberry mixture, tilting the glass slightly to preserve the bubbles and creating a lively, effervescent top.
Garnish each drink with a few whole cranberries or a twist of citrus peel for a festive touch and aromatic appeal.
Serve immediately with a straw or stirring stick, allowing guests to enjoy the bright color, fizz, and tart flavor of this sparkling cranberry cocktail.