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Start by marking all the wall studs. In most cases, wall studs are spaced 16 inches on center, or use an electronic stud finder (image 1) to precisely locate them.
If a wall is longer than the length of a single piece of molding, you’ll need to create an “invisible” lap joint (image 2) by mitering the end of the first piece and the beginning of a second piece with opposite (mating) 45-degree angles. When the pieces are joined, the seam will be less visible than a square-butt joint. Make sure to position this joint over a wall stud so that both ends can be nailed into the wood. Use wood glue and finish nails to secure the joint.
Where two chair rail pieces meet at an inside corner, one piece is cut square and butted into the wall, while the other piece must be coped to closely fit the intersecting molding’s profile (image 3). To make a coping cut, first miter the rail end at a 45-degree angle so that the cut is facing outward. Then use a coping saw to back-cut along the edge of the miter cut, closely following the molding’s profile (image 4). This takes some practice, but when done correctly the second chair rail piece will fit perfectly over the first, hiding the joint.
For outside corners, both intersecting chair rails are mitered to a matching 45-degree angle, then glued and nailed together (image 5). Because wall corners are not always square, some trial-and-error cutting and test-fitting may be necessary to obtain a precise fit. Use scrap pieces to first test each corner then adjust your miter angles as needed.
Begin the installation at an inside corner. Cut your first piece square so that it will butt tightly against the corner. Apply a continuous bead of construction adhesive to the back of the chair rail, then align the bottom of the molding with your level line on the wall.
Use a pneumatic nailer or hammer with 2-1/2 inch finishing nails to attach the chair rail to the wall (image 6). Insert the nails through the thick parts of the molding to prevent the wood from splitting. Place two nails, one high and one low, at each intersection with a wall stud, and be sure the nails hit the stud to firmly secure the rail to the wall.
Begin nailing at one end of the rail and work your way along to bend and straighten any waviness in the molding. Because the wall itself might not be perfectly straight, gaps may occur between the rail and the wall, especially between the studs. These gaps can be filled later.