How to include custom post type posts on a page?

You will have to add a new page template, which is a copy of the template that you already use for your pages.

Then simply add a new loop:

$portfolio_items = new WP_Query( array(
    'post_type'      => 'portfolio',
    'posts_per_page' => 5,
    // ''               => '', # Other arguments here
) );
if ( $portfolio_items->have_posts() )
{
    while ( $portfolio_items->have_posts() )
    {
        $portfolio_items->the_post();
        // Example output:
        the_thumbnail();
        the_content();
    }
    wp_reset_postdata();
}