Select a spot to install the deck bench. Measure a 36-inch span across the selected area, using a tape measure. Mark the end points of the measurement with a pencil.
Place one deck post mount on the deck surface at each pencil mark. Position them so that the outside edge of the mounts are against the pencil marks. The mounts should be parallel to each other.
Drive the screws included with the deck mounts into the decking boards at each mounting hole, using a screw gun.
Slide one end of each post into a post mount. Drive deck screws through the mounts into the posts at each mounting hole.
Center the 1-inch by 12-inch by 36-inch board across the posts. Drive four evenly spaced deck screws through the top surface of the board into the ends of the posts. Center an L-bracket underneath the seat at each joint with the posts. Drive the included screws that came with the brackets through the mounting holes and into the posts and board, respectively.