How to use them: the easier way is to simmer the hibiscus flowers in water for a few minutes then leave them to cool and infuse a bit more as they do.
I’d recommend you start with cold water for best results. You’ll see the flowers start to seep out colour almost immediately. Once it comes to a boil, simmer a few minutes, then leave to cool. You could add the sugar while it’s still warm, but you can add later.