Use a post hole digger to dig a hole to a depth that is at least one-third the total length of the fence post.
Mix quick-set cement according to the package directions. Set the 4-inch-by-9-foot corner post in the hole, and fill it to within 4 inches of the ground's surface.
Lay a 3-foot-long carpenter’s spirit level against the side of the corner post. Adjust the post until the spirit level bubble is positioned in the center of the readout gauge.
Brace the 4-inch-by-9-foot corner post temporarily in place while the cement dries by hammering a wood stake into the ground on each side of the fence post.
Use an electric drill to drive one 2-inch screw through the end of a 1-by-2-inch board into one stake. Place the 1-by-2-inch board on the side of the corner post. Adjust the corner post to the level position, and drive a screw through the board into the post.
Repeat the same process with the wood stake on the other side of the corner post.
Brace the 4-inch-by-9-foot corner post by cutting two 4-foot-long bracing posts that are 4 inch by 4 foot.
Dig holes 2 feet from the base of the corner post that are 18 inches deep and against the fence line on each side of the corner post.
Place one end of each bracing post in the hole, and lean them over to the upright post. Mark the angles on the side of the bracing posts where they intersect the upright posts.
Cut the bracing posts where you marked the angles. Position the cut ends of the bracing posts flush against the sides of the corner post. Drive four 5-inch-long galvanized fencing screws, 1/4 inch in diameter, into the bracing posts and through to the corner posts.
Fill the holes with quick-set cement to within 4 inches of the ground's surface, and fill it in with soil.