Combine dried hibiscus flowers and 8 cups water in a pot.
Simmer over medium heat just until warm (avoid boiling).
Steep off heat for 10-15 minutes until a deep red color develops.
Strain through a fine-mesh sieve into a pitcher, discarding solids.
Add sugar to warm liquid and stir until dissolved.
Chill to room temperature then refrigerate until cold.
Serve with lemon slices, mint, and additional lemon juice to taste.