Hammer stakes into the ground at the corners of your fence line. Tie a string tightly between the two stakes to mark the fence outline. Measure out the locations for the posts and mark each area with paint.
Dig the holes with the posthole digger. They should be deep enough to fit 1/3 of the post, plus 6 inches. Corner, end and gate posts should be 6 inches deeper. Holes should be around 10 to 12 inches wide all the way to the bottom. Fill the holes with 6 inches of gravel.
Set your corner and end posts first. Nail a 2x4 about 1/3 of the way up the post and nail the other end of the 2x4 to a stake in the ground. Repeat with another 2x4 on the opposite side of the post, in the opposite direction. These will keep the posts straight while the concrete is curing. Use the level to check that the posts are at a 90-degree angle. You must check two adjacent sides of each post.
Prepare the concrete and pour it into the holes, so that the concrete comes above ground level. Ensure the concrete is angled away from the post. Recheck that the posts are level. Allow the concrete to cure for one day.
Tie a string between the corner posts to help keep the middle posts in a straight line. Repeat Steps 3 and 4 to install the middle posts.