@thintin apologies for the delay on this. If you want to use a next and previous button only then things should be easy to get working. In your theme single.php you should using the following to create your internal page links.
$defaults = array(
'before' => '<div class="pagenav">',
'after' => '</div>',
'link_before' => '',
'link_after' => '',
'next_or_number' => 'next',
'separator' => ' ',
'nextpagelink' => __( 'Next Page' ),
'previouspagelink' => __( 'Previous Page' ),
'pagelink' => '%',
'echo' => 1
);
wp_link_pages( $defaults );
Some basic css would give you grey buttons
.pagenav a {
display: inline-block;
background-color: #A6A6A6;
color: #fff;
padding: 6px 10px;
min-width: 85px;
text-align: center;
border-radius: 5px;
text-decoration: none;
}
Finally the settings to add would be.
Posts Wrapper: .site-content
Post Wrapper: .post
Navigation Selector: .pagenav
Next Selector: .pagenav a
Hope that helps.
Thanks