Arrange two six-foot-by-two-inch tree sections with two-inch diameters and two three-foot-by-two-inch tree sections with two-inch diameters in a rectangle. Notch the tree sections at each joint using the utility knife so that the tree sections nestle in each other. Predrill two holes at each joint and attach the tree sections together with wood screws.
Set the assembled rectangle on top of four six-inch tree sections with four-inch diameters, with one six-inch section in each corner at the junctions. Predrill down through the joints into the six-inch sections, which will serve as legs, then attach them together with long wood screws.
Set one one-foot tree section with a one-inch diameter on the side of the assembled couch, in the corner with the base of the section against the rectangle. Predrill two holes through the one-foot section and into the rectangle, then attach with wood screws. Repeat in the other three corners.
Hold one three-foot-by-one-inch section along the inside of two of these one-foot sections near the top. Predrill two more holes at this joint and then attach them with wood screws. Repeat for the other armrest.
Follow the same procedure as the armrest to attach the back rest, using the three-foot-by-one-inch sections and the six-inch-by-one-inch section. Predrill each joint and attach firmly with wood screws.
Split two of the six-foot-by-one-inch tree sections in half using the table saw. Attach these half-tree sections to the front of the back support of the couch by predrilling the joints and attaching them horizontally with wood screws.
Split four three-foot-by-two-inch tree sections in half using the table saw. Attach them to the main rectangle with the flat ends down to serve as the cushion supports. Predrill the joints and attach them with wood screws.