Technical description

Start with a whole panel piece on the long side. Use 75mm screw or 3" nail.
Let the panel work overlap a corner according to sketch. Measure 85mm from the finished wall on the meeting wall top and bottom.
Draw a line between the marks and cut.
Meeting wall: Start with half a panel piece and repeat first wall procedure.
Now there is a hollowing in the corner, 85mm in both directions. Mount corner according to the adjoining sketch.