Take four of the 4-by-3 planks and arrange them in a square on the floor. Place two against the walls to check there is adequate clearance, and that you are not blocking doors or power outlet sockets. Scan the walls with a wire and stud detector first to ensure you do not drill into any wiring or pipes.
Join the four planks together in a square frame, driving a screw through the end of each plank into the one next to it. When you have a solid, sturdy square, take the remaining 4-by-3 planks and construct an identical square frame. Use the first one as a guide to make sure they line up correctly.
Place one of the square frames on the floor in the corner and screw it to the wall along the two sides that touch it. If your floor is suitable, it is a good idea to screw through the other two sides directly into the floor to give the frame more stability.
Take the two 1/2-inch-thick, 7-foot planks and stand them up against the wall. They need to be positioned at the two outermost corners of the square, either side of the corner of the room. Screw them to the wall and to the square frame on the floor.
Take the remaining square frame and hold it up against the wall against the top of the two vertical planks. You may need an assistant to hold it steady and straight for you as you screw it to the wall, and to the tops of the two planks.
Take the one remaining thick plank and stand it up in the corner of the frames that protrudes into the corner of the room. Attach it to the upper and lower square frames. Before attaching any outer panels, check that all the frame joints are square and secure. Insert additional screws if necessary.