Set the miter gauge at 45 degrees to cut the miters for the 14-inch by 2-inch by 3/4-inch top pieces.
Set the dado to "7/8 inch" on the table saw. Cut the bottom of each of the four top pieces of wood.
Glue the four top pieces together. Nail or screw them together to provide extra strength. Use a square to ensure they fit together accurately. Allow the glue to dry completely. Clamp the pieces, if desired.
Cut the bottom of the four side pieces at a 45-degree angle, just as you cut the top pieces. Hold the top and side pieces together to ensure they slide together. A little big of "wiggle" is OK, as you will secure them tightly.
Glue and nail the four side pieces to the four top pieces of the garden box.
Drill at least eight holes into the bottom piece for drainage.
Turn the box upside down and nail the bottom piece into place.
Caulk the joints with a silicone sealer.
Sand the garden box. Decorate it with several coats of outdoor paint or clear polyurethane, if desired.
Slide a plastic trash can liner inside the box, if desired.