The easiest way to paint a wall is to use a paint roller.
Start by loading the roller until the paint is evenly distributed but not dripping. Begin at one corner and make an M or W shape in a 3-foot section without lifting the roller from the surface. When you get to the end of the “letter,” lift the roller and reverse direction, filling in the unpainted areas. Take care to not spin the roller when you lift it from the surface.
Continue painting the wall above and below the section you have finished using the same M or W pattern. When you have finished painting the entire wall, run your roller, unloaded, along the four edges of the wall using one continuous stroke for each edge. This will blend the cut-in brush strokes from the roller marks.
Repeat this process for each of the remaining walls.