I've searched for "touch," "screen" and "touchscreen" but no relevant hits on my question. So ...

Where does one find out how to configure KDE to accept (correct) input from a USB touchscreen? I say "correct" input, because when I move my finger on the screen, the cursor is obviously responding, but not in the correct direction!

Under MS Windows special drivers have to be installed before connecting the USB cable. With KDE there is (at least!) response from the screen, immediately after installation of the OS.

Hardware details:
LG L1730SF, with USB cable for touch-screen input to computer.

You likely need to configure "X" correctly. Your distribution should usually provide utilities to do this, since it is outside KDE's scope.