Center one of the two crossbeams on the posts, check it for level, then tack in place with screws. Repeat for the other crossbeam (Image 1), making sure the beams are lined up (Image 2). With both beams held in place, drill two holes through the beam-post-beam "sandwich" at one end of the arbor. For each of these holes, slip a washer onto an 8-inch galvanized bolt, then fit the bolt through the hole. Add another washer and a nut, and tighten with a wrench.
Repeat the process to secure the other end of the beams.
In this project, the posts extend above the crossbeams until the crossbeams are bolted in place. When crossbeams are in place, use a reciprocating saw to cut off the top of the posts at the top of the crossbeams on either side.