This composite fence video is for surface mount installation. For in ground installation, it is the same, except you will bury the posts in concrete instead of using base plates.
Starting Your Fence At A Certain Height and Creating Gaps
To start your fence at a custom height (left above), simply cut a side cover to the desired size, slide the cut piece down the post channel and secure with a screw.
To create a custom gap (right above), simply cut a side cover to the desired size, slide the cut piece down the post channel and continue inserting rails and boards to create your custom look.