I would, first, have a general conversation with that person. Then maybe ask the person if he/she likes this game. I don't think Hangman is a bad game; in fact, I still enjoy it.
Sometimes a person might know the rules for Hangman, sometimes not. The word, "Hangman" might be misinterpreted as some kind of threat if the other person doesn't know the game. Ask the person if he/she knows the rules, and have that person tell you the rules.
Then you could start playing.
If it's a girl, don't tell her how much you like her. Just play the game with her.