Stand one board along the reveal line of the bottom board so the two boards are at 90-degree angle. Hold the two boards together with quick clamps until they are attached permanently
Use a pneumatic nail gun loaded with 1-1/2" pin nails and attach the three cut boards to make the beam. Drive nails from below so that the top board doesn't move out of place. Drive the nails at an angle to prevent the boards from pulling apart. Nail every 6" to 8". Use scrap wood from the earlier cuts to support the final board while you clamp it and then nail it into place. Fill the nail holes with painter's chalk.
The first beam should go in the center of your room. Use a laser line to guide the placement of the nailer board. The beam will be attached to this board (Image 1). Use a power nailer to temporarily hold the board in place. Then screw it in place with 3" deck screws. Place the finished box beam over the nailer board on the ceiling. Nail the beam into the nailer board on the ceiling. Nail the beam (Image 2) into the nailer board using 2-1/2" finish nails every 6" until it is very secure.