Measure the width of the wall on which you are installing the weatherboard with a tape measure.
Trim the weatherboard as necessary with a table saw. Often, the exterior walls of a building are longer than a single piece of weatherboard. This means that multiple pieces of weatherboard will be needed, and one of the pieces will have to be trimmed short to fit the wall.
Nail the weatherboard to the exterior wall of the building with galvanized nails and a hammer. Start on the bottom of the wall on which you are installing the board. Tap in nails along the top edge of the weatherboard every few inches apart. The nails do not have to be any set distance from one another.
Repeat steps 2 and 3 for the entire height of the wall, installing one horizontal layer over the previously installed layer. Overlap the horizontal rows by about a quarter of the height of each row. Use a sturdy ladder as you hit a point that is too high to reach from the ground.