Drop a post into a post hole or set it on top of a pier, depending on your construction method.
Drive temporary stakes into the ground with a sledgehammer so that the posts and the two stakes form a right triangle.
Clamp a length of scrap wood to one of the stakes. Make sure the scrap is long enough to reach about a foot past the post. The scrap wood should reach the post at an angle, allowing plenty of room to move the wood up or down as needed to plumb the post. Repeat the process for the second stake.
Strap a post level to the post. This type of level has a 90-degree corner and a strap that allows you to mount the device to the post, freeing up your hands as it measures plumb in both directions simultaneously.
Watch the bubble on the level as you move the post back and forth, front to back until the bubble ends up between the scribed lines on the level. When the post shows plumb in both directions, clamp the lengths of scrap wood in place.
Remove the temporary stakes and scrap wood bracing once the post is attached to a pier or after it has been set in concrete.