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Depending on your wall height, measure down from the ceiling 12 to 18 inches. Make a mark about one inch from each end of the wall. Snap a chalk line between the marks.
Next, measure the wall and cut out the pieces of the picture rail to length. If the wall length is longer then the molding, miter the two pieces together. Use a 45-degree cut on both sections of molding for the miter.
Apply a bead of construction adhesive to the back of the picture rail, then align the bottom of the picture rail with the chalk line you made earlier.
Attach the molding using a pneumatic nailer and 2 1/2-inch brads. Put in one nail about every six inches, making sure the molding aligns with the chalk line. Use a nail set to for any nails that are sticking up; fill the holes with a dab of paintable caulk. Apply a bead of caulk where the picture rail meets the wall.