1. Muddle (squish together) the lime juice using a muddler (or a rolling pin if you don't have a muddler), syrup and mint in the cocktail shaker.
2. You can either buy pre-mixed sugar syrup or if you want to make your own
3. Pour in the rum, Angostura Bitters and ice
4. Shake everything well (don't forget the lid!)
5. Strain into a cocktail glass and top up with champagne
6. Garnish the drink with a mint leaf