Use a tape measure to mark the desired height of the top of the chair rail on the wall on both sides of the corner. Use a yardstick to draw a horizontal pencil line at this height.
Measure the length of the line that goes around the corner to determine the length of molding you will need.
Place the molding in a miter box and saw it to the correct length with a jigsaw. Cut the molding 1/4 inch too long.
Place the molding on the wall to check the length. Shave the ends with a wool file until the molding is the exact length you need.
Use a caulking gun to apply a line of construction adhesive along the middle of the molding.
Place the molding on the wall. Hammer a trim nail into the molding every 15 inches along its length. Position one nail in the top groove of the molding and the next nail in the bottom groove. Alternate the position of the nails along the molding. Angle the nails toward the center of the rail.