Measure the vertical height of the wall opening where you are planning to put the decorative wood post.
Measure the wood post and be sure it is the right size for the opening. Trim the post if necessary so that it is 1/16- to 1/8-inch less than the opening with a circular saw.
Position the post in the opening. Check it with a 4-foot level to be sure it's perfectly vertical by holding the level against the side of the post in several different locations.
Tap wood shims in between the post and the floor to secure it in place. Permanently secure the post by driving 4-inch screws at an angle through the base of the post into the floor and through the top of the post into the header over the opening.
Cover the heads of the screws with wood filler then paint or stain the post to match the decor of the room as desired.