Decide how many shelves you want in your custom bookcase. Also, determine the width of your bookcase.
Purchase the lumber. Pine or fir works. Buy enough 1-by-10 lumber for the number of shelves you have chosen.
Measure your shelf spacing on each side. Allow 3 inches from the floor for the lowest shelf. Measure your tallest books, and allow space for them on the bottom shelf.
Cut the two side pieces. Using the dado head cutter attachment on your table saw, cut a ½-inch deep by ¾-inch wide groove to hold each shelf.
Allow 3 inches extra for the top shelf piece as it fits over the two side pieces. Cut the remaining shelves.
Put the shelves in place. Either nail them in or use wood screws.
Lay the bookcase on its front. Measure the back. Cut the plywood to fit. Nail or screw it in place.
Sand the wood as needed. Stain and finish your bookcase as desired.