Bring hibiscus flowers with sugar and 1⁄4 l water to the boil in a pot. Simmer for 10-15 minutes until it is slightly thick. Remove from the heat, pour through a sieve and collect the syrup (makes about 200 ml). Let the syrup cool down.
Spread the syrup with ice cubes over four glasses. Fill up with lemon juice, rum and mineral water. Serve immediately.