Place a deck post mount over the top end of the deck post. Drive the screws that were included with the deck post through the mounting holes into the deck post with using a screw gun.
Insert one end of the 4-by-4-by-16-inch post into the deck post mount. Drive the screws included with the deck post mount through the mounting holes into the post.
Fit the four post trim pieces from the trim kit around the post so that it is centered over the seam between the existing deck post and the added post. The trim pieces fit together around the post like a picture frame. Hammer finish nails through the trim pieces every two inches.
Center a nail set over each finish nail and tap it with a hammer approximately 1/16-inch below the surface of the wood. Fill the holes with wood putty using a putty knife. Allow the putty to dry for 24 hours.
Sand the wood putty flush with a fine-grit sanding pad. Wipe off the surface of the wood with a tack cloth.
Apply wood finish to the trim pieces and post extension with a paintbrush. Allow the first coat to dry for four hours and apply a second coat. Repeat Steps 1 through 6 for each deck post you would like to make taller.