A method for mounting a guide rail in an elevator system elevator shaft first carries out mounting steps by a mounting device having a carrier component, an installation component and a control apparatus controlling the installation component. The control apparatus determines, on the basis of an actual position of the mounting device and a desired position of a guide rail to be installed, a position of a marking on a shaft wall of the elevator shaft relative to the actual position of the mounting device. Based on the marking, an installation step determines the desired position of the guide rail. The marking is applied to the shaft wall by the installation component. The previous steps are followed by the steps of alignment the guide rail in relation to the marking and fastening the guide rail to the shaft wall, which steps can be performed manually by an installer.