A sill is aligned in the vertical direction on a laser line. The alignment of the sill in the horizontal direction is undertaken with a line set of an assembly jig. The line set includes at least two lines, for example a first line and a second line. The first line runs by a first run from the first part of the assembly jig to a third marking of the first line and from there by a second run to the second part of the assembly jig. If the lines are tightened and the marking is at one corner of the sill and the free end of the second line at the other corner of the sill, the sill is aligned in horizontal direction.