Dig a hole with post hole diggers that is three times the width of the wooden fence post. Dig the hole 12-inches across, for example, when working with a 4-by-4 post. Dig the hole at least one-third the length of the total post length.
Pour 4 inches of gravel in the bottom of the hole and tamp it flat with the handle of the shovel. Place the wooden fence post in the hole so that it is centered as much as possible.
Square the post so that one side is parallel to the direction in which the wire, pickets or panels will run. Stand to the front of the post and drive a 2-foot 1-by-2 stake into the ground 1-foot or so out from the post with a hammer. Align the stake with one side of the post. Move to the side of the post and drive a second 1-by-2 stake 1 foot or so out from the post so it is in line with the front or back of the post.
Attach a 3-foot length of 1-by-2 to the two sides of the post that align with the 1-by-2 stakes. Use a drill and driver bit to secure the 1-by-2 to the fence post with one wood screw each.
Place the level on one side of the fence post and secure the 3-foot 1-by-2 to the stake to which it aligns. Use the drill and driver to secure the 3-foot length with a wood screw. Level another side of the wooden fence post and secure the remaining 3-foot 1-by-2 to the second stake with a wood screw.
Pour quick set concrete into the hole up to a point that is 3 to 4 inches below the surface of the ground. Add water to the concrete following the manufacturers directions. A rule of thumb is to add enough water to wet the concrete without making it too thin.
Allow the quick set concrete to dry and thoroughly cure overnight. Remove the 1-by-2 supports and back fill around the post with dirt that was removed when digging the hole. Use a shovel to move the dirt and tamp it down around the post with the shovel or your foot.