Best process I could find so far - Assuming Neon for Plasma 6.0 - is to install Synaptic and then either install software-properties-qt and then run pkexec env WAYLAND_DISPLAY=$WAYLAND_DISPLAY XDG_RUNTIME_DIR=$XDG_RUNTIME_DIR QT_QPA_PLATFORM=wayland software-properties-qt from the main menu or krunner; or just search for and install the relevant nvidia-driver-VERSION package using synaptic.