Set Crossbeams and Add Trim
Position one 2x12 crossbeam across the front of the two posts, level it to the top of the posts and temporarily hold it in place with a screw at each end. Repeat the process for the other beam, sandwiching the posts. Make sure the beams are level and lined up with each other.
With the beams in position, drill a hole through the beams and post. Slip a washer over a rust-proof carriage bolt, insert it through the hole, then secure it with a washer and nut. Tighten with a socket wrench. Repeat for the other end of the crossbeams.
To add visual interest, screw short pieces of 4x4s to the top of the crossbeams. Place the trim pieces 6 to 12 inches apart, perpendicular to the beams.