A quick alternative to MrE's technique: rather than shaving lots of material off the wide face of a batten to make it fit perfectly to the wall, you could just pack it our behind with scrap/shims to make it vertical and screw it to the wall (i.e., as in MrE's diagram, but then just leave it there). Then cover up all the scrap/shims, by shaping a thin (6mm) bit of wood that you then nail to the side of the batten i.e., the face that faces the viewer in MrE's diagram. Essentially you'd be boxing in the scrap behind the batten. Add a bit of painters mate/caulk and a slick of paint and it will look neat enough.
A quick alternative to MrE's technique: rather than shaving lots of material off the wide face of a batten to make it fit perfectly to the wall, you could just pack it our behind with scrap/shims to make it vertical and screw it to the wall (i.e., as in MrE's diagram, but then just leave it there). Then cover up all the scrap/shims, by shaping a thin (6mm) bit of wood that you then nail to the side of the batten i.e., the face that faces the viewer in MrE's diagram. Essentially you'd be boxing in the scrap behind the batten. Add a bit of painters mate/caulk and a slick of paint and it will look neat enough.