Drive the corner post. Use a post driver, a weighted steel tube placed over the post and worked up and down to apply pressure to the post. Drive the post into the ground until the flat metal extensions near the bottom of the post are underground.
Place a second post a distance equal to the length of the post, minus about one foot, down the fence line. Drive this post using the post driver in the same process as in step 1.
Attach a commercially available bracket to each end of a third post. Attach a similar bracket to the second post near ground level. The brackets are available from various commercial sources and have different mounting systems, generally using bolts to hold the bracket in place.
Connect the brackets attached to the third post to the bracket at the base of the second post. Place the other end of the post against the corner post. The end should rest about six inches from the top of corner post. Attach a bracket to the corner post at this point. Connect this bracket to the bracket at the end of the third post.