Assemble bookcases if needed. Paint all units the same color—we recommend a darker finish. Once you have the bookcases set in place, screw them together (eliminating gaps between the pieces) and anchor to the wall.
Add trim to the top of the unit—decidedly the most important step when it comes to creating custom, built-in appeal. Our pièce de résistance was created by layering a 3/4 x 2 5/8-inch piece of painted poplar trim over the top of a 1/2-inch x 1 1/2–inch piece of trim (length will vary based on your bookcases). Cut the trim to size and miter the corners (cut meeting ends at a 45-degree angle) for a precision fit. Sand the trim before painting the same color as the shelving units.
Nail the smaller trim across the front of the entire unit using trim nails. Use nails shorter than the thickness of the width of the molding and bookcase panel together to avoid poking through the inside of the bookcase. Make sure the trim pieces are level to the top of the bookcase.
Repeat with the second board, setting it higher than the first so it reaches to the ceiling. Hold it tightly to the ceiling when nailing in place to avoid gaps between the unit and the ceiling and to keep the trim level.
Fill holes with wood filler, and touch up with paint. Try to find a wood filler as close in color to the bookcases as possible to avoid obvious touch-ups. Fill the shelves with your favorite books and accessories. Also experiment with paint and contact paper inside the shelves for a fun custom look.