Take the screenshot you already know how to take. That is the whole instruction — a phone keyboard genuinely cannot read the conversation above it, so SayThis reads the picture instead, on your phone. You can also tap to use a message you copied, or type the point you want to make and let it write around that.
It reads the thread: who said what, in what order, in whatever language it is in. Then it writes one reply — in Auto, which picks the voice the message needs, or in whichever of the sixteen you tap. One at a time; choosing a tone drops the last one.
If it is not right, reshape it without going back: Shorter keeps the point and loses the words, Another writes a genuinely different reply rather than the same one reworded, and Tone rewrites it in a different voice. Undo walks it back if you preferred the one before.
Then Insert puts the draft in the message box — the real one, in the app you were already in. Edit it, and send it. SayThis never sends anything itself, and it never will.