Measure the length and width of each wall section in the stairwell. Write these figures down.
Draw each wall shape on a sheet of cardboard with a pencil, using your measurements, to create a template. Cut out the template with a utility knife.
Place the cardboard template onto the back of a wall panel and trace the shape a pencil.
Trim the wall panel with a circular saw, following your trace lines as you feed the wood through the saw. Do not rush the paneling through the saw and use a new blade to avoid creating a jagged edge.
Fit the cut wall panel into place on the stairwell and nail it into the wall studs with a hammer, placing one nail every 12 inches vertically.
Repeat Steps 2 through 5 for each wall section.