It’s very simple.
You understand what’s going on, right? It’s the other guy that’s an idiot.
Well then. How do you talk to someone who doesn’t understand something as well as you do?
Well, what’s the classic example of someone who doesn’t understand things very well?
I’ll just tell you — it’s a child.
And how do you explain things to a child?
Patiently. Calmly. If possible, lovingly.
But what if they don’t want to listen even then? In that case, you switch tack. You let them talk themselves out. The less pushback you give them, the quicker they’ll run out of steam.
At that point, you calmly, gently ask them questions until they reveal their confusion.
If, by this point, you have managed to exude a genuine concern and desire to enlighten, then maybe they’ll be willing to listen to you.
If they still don’t, then get out — you’re just giving yourself an ulcer.