Finish the trim with either wood stain or paint. If staining, rough up the surface with a fine-grit sandpaper. Wipe this clean with a tack cloth. Apply one coat of stain and let it dry. Rough up the surface again and apply one coat of polyurethane. Let this dry. If painting, sand first. Prime the trim and let the primer dry for four hours. Apply one coat of paint over the primer.
Find the studs at the top of the wall with a stud finder. Mark where they are with a pencil.
Nail up the first corner piece with a hammer and finishing nails. Attach one nail on either side of the corner piece.
Nail up the first strip of trim with a hammer. Align the trim with the corner piece. Insert one nail per stud. Continue until you get to the next corner. Nail up the next corner piece, then the remaining trim.