All of the eight posts are now firmly set in place but the tops need to be trimmed off. (Remember, all the posts were longer than needed).
Measure from the ground 87" up one of the posts (we are assuming the ground is reasonably level) and mark with a pencil.
With your set square, draw a pencil line around all four sides of the post (87") from the ground).
You will probably need a ladder or saw stool to do this.
Now, hammer a 4" nail partly into the outside edge of the post you have just marked on the underside of the pencil line.
Sit one end of a level on that nail and hold the other end level against the next post.
Mark the underside of the level with a pencil on the second post, checking of course, that the level is level, too.
If your level is not long enough to span from one post to the next, get a straight piece of lumber 40" long, and use that with the level sitting on top.
With your set square mark a pencil line around all four sides of the second post, and again hammer a 4" nail partly in, the same as on the first post.
Repeat this action until all eight posts are marked.
Check that the pencil line on the last post is level with the line on the first post, if not, go around again with the level and find the discrepancies.
Cut off the top of the posts where you have marked, with a circular saw.
If the circular saw does not cut all the way through the post, finish the cut off with a handsaw. Make sure you have firm footing.
If necessary erect a scaffold about 2ft off the ground to help you reach.
For the head beams you will need eight pieces of 4x4 lumber 40" long.
Pencil a square line at one end of the beam with your set square as shown in the diagram below and then pencil another line 22.5 degrees from the first.
Cut on this line with your circular saw.
If your circular saw does not cut all the way through the beam, finish the cut off with a handsaw.
Measure from the long point and along the beam 36 1/4" and do the same cut that end as you have done the other end.
Another cut is required: 2" long and square (right angles) to the edges just cut.
To achieve this, hold your set square against the newly cut edge and slide it along until you're able to pencil a line 2" long, square to the cut edge and finishing at the beams longest (face) edge.
Sounds complicated I know, but just follow the drawing in the circle. Also see fig 4.
Repeat this for all eight beams.
You now have all eight beams ready to sit in place.
Sit the eight head beams on top of the eight posts as per fig 4 and skew nail (toe nail) the beams to the posts with 4" galvanized nails.
Nail a 12" long metal strap plate to the top side of each head beam join. Also see fig 4.
This will stop any likely outward movement of the gazebo.
Nail the metal strap plates with 2" (flathead) galvanized nails.