How to Build a Jackleg Fence
Stand one set of support posts upright, the "X" centered on your fence line. Set the end of one rail in the cradle formed by the top of the "X." Attach the rail with a 50-penny nail. Move to the other end of the rail. Stand up a second set of support posts and fit the end of the rail into the cradle of the "X" of the second rail. Secure with another nail.