Take 2x4 and place at top of the wall where it meets the ceiling.
Nail it to the wall so it is fully secured. (parallel to the ceiling)
Repeat this process, taking the remaining 2x4 and nailing it parallel to the wall where the wall meets the floor.
Take a wooden slats and secure it to the top 2x4 that is attached to the wall/ceiling.
Use a level to make sure the slat is straight and secure the bottom of the slat to the 2x4 that runs along the floor.
Measure desired distance between slats, and secure the next piece to the wall at the ideal space. *TIP: You can use a piece of precut wood as a spacer when placing planks to ensure even spacing
Once wall is fully secured and mounted, paint, stain the wood (or leave natural).