Adjust the arm of the miter saw to cut a 45-degree angle and lock into place. Lay each board, one at a time with the 8-by-12-inch side flat on the table of the saw. Cut each end of the 1-by-8-by-12-inch boards at a 45-degree angle. One 8-inch side will measure 12 inches long, and the opposite side will be shorter. The 12-inch-long side will have the long points on the ends. These are the side boards.
Fit the side boards together to form a box. One 1-by-12-inch edge of each board will rest flat on the work surface. The sides of the boards with the long points will be facing out. The boards will fit together like a picture frame.
Fasten the box together by driving two evenly spaced wood screws through the outside edges of the sides into the ends of the adjoining sides at each corner joint.
Place the 1-by-4-by-12-inch boards across the top of the box. Space them evenly across the box. There will be small spaces between the boards because boards do not actually measure the width they are called. For example, 4-inch boards are actually 3 1/2 inches wide. The spaces between the boards will provide drainage for the flowerpot.
Drive two evenly spaced wood screws through the boards into the edges of the box, using the screw gun. Turn the box over and upright. Slide a plastic liner into the opening.