How to add post meta fields to an article’s as meta elements

In the post edit screen you can add the data as custom fields and then pull those custom fields into your post templates, like this:

If you’d like a better admin interface for these fields, that’s possible and there are plenty of guides around the web and on this site. Search for admin metabox.

To pull the data into your theme, add this untested code to functions.php or if you can’t edit the theme make it into your own plugin.

function wpse_235005_post_meta(){

    if ( !is_single() ) {
        return; 
        /* bail out if not showing a single post of the right type
           You may want to adjust this conditional to suit your site,
           maybe checking for a custom post type
        */
    }

    global $post; // outside the loop make sure we can see this post

    $fields = array(
        "citation_publication_date",
        "citation_journal_title",
        "citation_volume",
        "citation_issue",
        "citation_firstpage",
        "citation_lastpage",
        "citation_pdf_url"
    )

    foreach( $fields as $field ) {

        if( $value = get_post_meta($post->ID, $field, true) ) {

            // in PHP empty strings are falsy

            echo '<meta name="';
            echo $field;
            echo '" content="';
            echo $value;
            echo '">';
        }
    }
}

add_action('wp_head', 'wpse_235005_post_meta');
/* adjust the priority to move your meta elements up or down 
   within your document's head
*/