Stand up two 3-by-3-by-12-inch boards, 38 inches apart and parallel to each other. Have an assistant hold a 1-by-8-by-38-inch board standing on its long, thin side between the two posts, to serve as the footboard, with the front of the footboard flush with the front surface of each post.
Position a 4-inch metal corner bracket between the footboard and each post. Drill holes through each hole in the corner bracket, into both the footboard and the posts, taking care not to come out the other side of any of the boards. Attach the corner bracket to the footboard with 3/4-inch wood screws and to the posts with 1-inch wood screws.
Repeat Steps 1 and 2 with an additional two posts, a 1-by-24-by-38-inch headboard and two more metal corner brackets, placing the rear of the headboard flush with the rear of the posts. Tighten them securely with the same screw attachments.
Position the headboard and footboard parallel to each other and 75 inches apart. Hold a 1 1/2-by-3 1/2-by-77-inch board between the two, with its largest side flush up against the inside surface of two of the posts, on the same surface where the metal brackets are installed. Drill three holes through the inside of the long board, which is the mattress support board, and into each of the posts. Set 2 1/2-inch wood screws in the holes, and fasten the boards to the posts. Repeat for the other side with another long board.
Position six evenly spaced 1 1/2-by-3 1/2-by-38-inch boards across the tops of the mattress support boards, with their large surfaces flush against the mattress support boards. Drill two holes through the top of each board into both mattress support board, for a total of 24 holes. Set a 2 1/2-inch wood screws in each hole, and tighten them securely.