Measure 30 inches from the long side of the 4 feet by 8 feet backer-board. Backer-board is thin (1/8 inch thick) composite board used for a variety of applications. The measurement should be so that when cut, there is a 30 inch by 8 foot section. Cut the chair rail to an 8-foot section.
Position the backer-board on the bottom half of the wall. If there is floor molding, the backer-board can rest on the top of the molding. If not, position it all the way to the floor. Use small nails or the nail gun to attach the backer-board to the wall.
Position the chair rail to sit on top of the backer-board and secure it to the wall with nails. Repeat this for the entire wall to be modified.