In a self-propelled construction machine comprising a machine frame with a longitudinal axis, a controller for the travelling and milling operation, a height-adjustable working drum, and a slewable transport conveyor: that the control system, at least as a function of a virtual trajectory for positioning the transport conveyor which is freely specifiable in a stationary coordinate system that is independent of the position and alignment of the machine frame, controls, by means of open-loop control or closed-loop control, at least the slewing angle of the transport conveyor automatically in such a fashion that a reference point of the transport conveyor always remains on the specified trajectory in the case of a change in position of the machine frame within the coordinate system.