Measure the desired height of the chair rail on each step and make a light marking on the wall. Connect each mark with a straight line to use as a guide when installing.
Use a stud finder to locate the wall studs along the installation line on stairway wall. Mark each stud with a light pencil “x” so you know where to place the nails.
Attach the chair rail by holding the section along the marked line and pounding a finishing nail though the chair rail and into each of the wall studs along the stairs. Continue to add chair rail sections in this manner to complete the stairway attachment at an angle.
Seal the top edge of the chair rail, where it meets the wall, with a line of caulk. Smooth and fill in the joints between each chair rail section with wood putty. Wait for the caulk and putty to dry before painting.