Contact your county building code enforcement office to find out how deep your fencepost must be buried to prevent frost heave from moving it when the ground freezes. Designate one end of the fencepost as the bottom. Measure from the bottom of the post to the required burial depth and mark the post with a line to indicate ground level. Measure from the ground-level line to the desired height of the installed fencepost. Mark the post at this level. Use a carpenter's square to draw a perpendicular line across the post at this level.
Cut the fencepost across the perpendicular line. To prevent confusion, mark the top of the post with a "T" and the bottom of the post with a "B." Use a post-hole digger to dig the post hole 6 inches deeper than the portion to be buried. The hole should be 2 and 1/2 times as wide as the post and flared slightly at the bottom. Pour gravel in the hole to bring the depth of the hole up to the length of the buried portion of the post. Tamp and level the gravel with the handle of a shovel and recheck the depth of the hole. Add more gravel and tamp, if necessary.
Position the bottom of the post in the post hole. Fill the hole about half-full with dry concrete/sand mix. Add about one inch of water and tamp to remove bubbles and to settle the concrete. Use a spirit level to plumb the post on two adjacent sides. Fill the hole with concrete and add another inch of water. Tamp and level the concrete. Recheck the post to make sure it is still plumb and adjust its position if necessary. Allow the concrete to set, as recommended by the manufacturer, before attaching rails to the post.