Digital models of the application areas of a left-ear and of a right-ear hearing device are established. The models (140r# and 140l#) are displayed on a computer display. The location of the model (SP#) of the sagittal plane of individual's head is estimated with respect to one of the two digital mold models. Then that one digital model (140r#) is digitally mirrored at the digital model of the sagittal plane (SP#) resulting in a mirrored digital model (140m#). Then the other digital model of the application area (140l#) and the mirrored digital model (140mr#) are brought into best possible covering alignment (140′l#). Units to be positioned in predetermined mutual relationship are applied to the models in best possible alignment.