Complexity of a process of mounting a cushion (40) to an air passage element (20) of a breast pump device is decreased by only requiring a user to arrange the cushion (40) on the air passage element (20) in a first arrangement in which the cushion (40) is easily accessible. Particularly, the user does not need to perform cumbersome actions aimed at accurately putting the cushion (40) to an intended, correct position, as a predetermined mounting arrangement of the cushion (40) on the air passage element (20) is realized under the influence of underpressure prevailing at a back side of the air passage element (20). In the process, the cushion (40) is put to the predetermined mounting arrangement, wherein the cushion (40) is made to flex and/or move.
