In a large pot, pour in 4 cups water and add hibiscus flowers. Bring to a boil and cook for 2 minutes. Turn off the heat, cover and let it steep for 30 minutes.
Strain the mixture into a fine sieve over a large bowl or measuring cup; discard the flowers. Add sugar to concentrate Agua de Jamaica and stir well before adding 4 cups cold distilled water. Stir well, taste and adjust if necessary, transfer to the fridge to chill, about 4 hours.
Pour Agua de Jamaica in a tall glass over ice cubes. Makes 7 cups.