Measure across the front of the bookcase at the top, sides and bottom and write down these measurements.
Cut your molding to correspond to your measurements. Miter the corners, which will involve inward-angling cuts that are usually 45 degrees. Use the miter box and miter saw to get the proper angle.
Apply wood glue to the back of the piece of molding that fits at the top of your bookcase, and press the molding in place. Drive finishing nails into the ends and middle of the molding to help secure it.
Glue and nail molding along the sides and bottom of the bookcase. Wipe away any excess glue with a damp rag.