Open body language is a big part of it. Or generally gestures that clearly communicate you’re not upset - smiling, gentle/playful touches.

Also if you are angry but it’s at a situation and not a person, make that clear. E.g. even something as simple as the difference between “this totally sucks!!” and “this totally sucks, doesn’t it?” The second options conveys that you’re experiencing an annoyance together, as opposed to you experiencing an annoyance and you may decide to take it out on them.