Watch the video of this step.
After the concrete in the post holes has set, and the beams and rafters are cut, cut the tops of the posts to an even height. For our 8-foot tall pergola we cut our posts to measure 7-1/2 feet from the ground. The rafters on top will eventually add another six inches to the height (image 1 below).
Measure, mark then cut the tops with a circular saw. As you cut each post, you can use a 2x4 and a level to make sure all the posts are the exact same height.
Install the beams. Each beam is two 2x8s fastened together on either side of a post - an inner and outer beam. Begin by resting the first 2x8 beam on-center over a pair of posts to make sure the overhangs are of equal length. Measure each as you go along (image 2).
Secure each end of the beams to the top outside face of each post with two 4-inch screws to temporarily hold them in place. Attach the rest of the beams in the same way.
All that's left is to install the rafters and to permanently secure the beams in place with the bolts. Lay out the rafter locations on the tops of the beams at one-foot intervals.
Take time to measure the overhang of each rafter carefully to keep the whole set in alignment (image 3). Install the rafters by fastening each in place with two 5-inch galvanized drive screws at each end.
With everything in perfect alignment and centered you now can permanently secure the beams to the posts using 8-inch galvanized bolts. Using a 1/2-inch drill bit, drill a top and bottom pilot hole centered on each post. Use a hammer or rubber mallet to pound the bolts through the beams and post. Finally secure them using a 1/2-inch nut on the inside of the structure.