How do you say something to someone you want to threaten without it being legally extortion or a threat?

You dont, the best thing to do is to keep your mouth closed and not say anything to them. Or if you want to tell them something, tell them how you feel about what they did. Leave it at that to stay legal. I hear what you say loud and clear - what if I said how would you feel if I did this? If that is all that you said and you left it at that then you are clean and clear.