Dig a hole for the fence post with post hole diggers. Make the hole at least half the length of the post that will be exposed above ground. For example, dig the hole 3 feet deep if you want a 6-foot length of post extending above ground.Make the hole twice as large as the post diameter.. Place 3 to 5 inches of pea gravel in the bottom of the hole. Set the post in the center of the hole and wiggle it slightly to settle the bottom of the post into the gravel.