Dig 4-foot holes for the arbor posts. Position the holes so that the holes on the sides are 3 feet apart. Then ensure that the holes on one side are 4 feet from the holes on the other side.
Fill in the holes with 6 inches of gravel, and pack it down.
Place the posts in the holes. Tamp down 3-inch layers of gravel and dirt until the holes are filled around the posts. Check to ensure they are level as you go.
Set the 6-foot-long 4-inch by 4-inch post on top of the 10-foot posts so that they run side to side across the 4-foot-wide opening between the posts. Place one on the front two posts and one on the back two posts.
Drill holes in the top of the posts to connect the crossbeams to the vertical posts. Drill two holes at each post connection. Use a wrench to twist two 5-inch bolts into each location where the crossbeams touch the vertical posts.
Cut the ends off the 6-foot-long 2-inch by 6-inch boards to a 45-degree angle using a circular saw. Mark the cuts with a pencil first to give you a guideline to follow. Cut the boards on both ends so that the 45-degree angles on each end mirror each other.
Place the cut boards at a perpendicular angle to the cross boards at the top of the arbor frame. Put one on each side and then one every foot. Set each on its 2-inch side so that the longer end of the 45-degree angle is on top.
Screw the sides of the 2-inch by 6-inch boards into the 4-inch by 4-inch crossbeams with 3-inch screws. Screw into the side of the boards at an angle so that the screw goes into the crossbeams.