A method for providing an opening in an inner wall of a building structure for an interior lining (94; 96, 98) for connection to a window frame of a roof window, comprising the following sequence steps of: preparing an uncovered portion of an exterior of the roof or wall substantially matching the dimensions of the window, providing a marking tool with a window engaging member having a first surface adapted for abutment with the window frame, and a marker holder member having angle indicating means and a marker, the marker holder member comprising a marker locking mechanism with a longitudinal axis, in which the window engaging member and the marker holder member are pivotally connected through a pivotal connection allowing adjustment of an angle (a) between a plane defined by the first surface of the window engaging member and the longitudinal axis of the marker locking mechanism, the angle indicating means being configured to indicate the angle (a) of the axis of the marker locking mechanism when the marking tool is abutted against the window, and the marker being configured to indicate an intended cutting location, the window engaging member of the marking tool being provided with at least one leg providing said first surface and configured to abut the window frame in a mounted condition on the roof window, and said at least one leg of the window engaging member being removably attachable to the window frame in said mounted condition, removably attaching the marking tool against the window, placing the window on the uncovered portion at the exterior of the roof or wall and the angle indicating means being utilized to ensure that the marker holder member is at a predetermined angle, marking an intended cutting location by means of the marker and preparing an opening through the wall or roof at the intended cutting location. preparing an uncovered portion of an exterior of the roof or wall substantially matching the dimensions of the window, providing a marking tool with a window engaging member having a first surface adapted for abutment with the window frame, and a marker holder member having angle indicating means and a marker, the marker holder member comprising a marker locking mechanism with a longitudinal axis, in which the window engaging member and the marker holder member are pivotally connected through a pivotal connection allowing adjustment of an angle (a) between a plane defined by the first surface of the window engaging member and the longitudinal axis of the marker locking mechanism, the angle indicating means being configured to indicate the angle (a) of the axis of the marker locking mechanism when the marking tool is abutted against the window, and the marker being configured to indicate an intended cutting location, the window engaging member of the marking tool being provided with at least one leg providing said first surface and configured to abut the window frame in a mounted condition on the roof window, and said at least one leg of the window engaging member being removably attachable to the window frame in said mounted condition, removably attaching the marking tool against the window, placing the window on the uncovered portion at the exterior of the roof or wall and the angle indicating means being utilized to ensure that the marker holder member is at a predetermined angle, marking an intended cutting location by means of the marker and preparing an opening through the wall or roof at the intended cutting location.