Place two 1-by-12-by-18-inch boards parallel to each other approximately 12 inches apart with the 1-by-12-inch ends flat on a work surface. The 12-by-18-inch sides should be parallel and facing each other. Insert one 1-by-12-by-18-inch board between each end of the boards to form a box. This is the planter box assembly.
Put together a second planter box assembly in the same manner as Step 1. There are two planter box assemblies total for the project. Drive stainless steel wood screws through the joints of the planter boxes to fasten them together. Use a screw gun and space the screws every 3 inches along the joints.
Align the 2-by-4-by-12-inch board with the 4-by-12-inch side flat against one end of each planter box assembly so that the top edge of the board is flush with the top edge of the box. Fasten the board to the bottom of each assembly with stainless steel wood screws. Space the screws evenly every 3 inches. These are the seat support cleats.
Space the planter box assemblies 36 inches apart from the inside edge of the ends with the cleats facing the center. Align the 1-by-12-by-36-inch board between the two assemblies so that it rests on top of the 2-by-4-by-12-inch seat support cleats. This is the seat board. The 1-by-12-inch ends should be flush with the outside edge of the planter box assemblies.
Drive stainless steel wood screws with a screw gun through the top of the seat board into the support cleats. Space the screws evenly every 2 inches.
Slide a planter box liner into each top opening in the two planter boxes.