Thread the outlet through the cutout, and secure the panel to the wall. Then secure the outlet back in place. Measure and cut chair rail molding to the length needed for each wall. If you have to join two lengths of molding on a long wall, be sure to miter the joints. You should also miter any corner joints.
Mitered joints are stronger and look better than butt joints.
Attach the molding to the top edges of the panels, and secure the molding to the wall with finish nails. Drive the nails until the heads are just above the surface of the molding, and then use a nail set to finish driving them just below the surface. Fill the holes with wood putty or caulk. Reattach the base molding, replace the outlet covers, and turn the power back on.