Place the champagne flute in the freezer for 10-15 minutes prior to assembly to ensure maximum temperature retention.
Tightly hold the chilled flute at a 45-degree angle.
Slowly pour the dry sparkling wine down the inside wall of the glass to prevent excessive foaming and preserve carbonation until the flute is half full.
If using orange liqueur, add it slowly to the glass now.
Top the glass with the chilled orange juice. The density of the juice will cause it to naturally mix with the wine; avoid stirring to keep the bubbles active.
Garnish the rim with a fresh orange slice and serve immediately.