1. Navigate to the WordPress plugin directory of your development installation.
2. Navigate to the ch10-book-review-widget directory and edit ch10-book-review-widget.php.
3. Find the Book_Reviews class and add the following block of code within the class to define the widget method:

function widget( $args, $instance ) { 
    if ( 'true' == $instance['render_widget'] ) { 
        // Extract members of args array as individual variables 
        extract( $args );
 
        // Retrieve widget configuration options 
        $nb_book_reviews = 
             ( !empty( $instance['nb_book_reviews'] ) ? 
                       $instance['nb_book_reviews'] : 5 ); 
 
        $widget_title = ( !empty( $instance['widget_title'] ) ?  
                          esc_attr( $instance['widget_title'] ) : 
                          'Book Reviews' ); 
 
        // Preparation of query string to retrieve book reviews 
        $query_array = array( 'post_type' => 'book_reviews', 
                              'post_status' => 'publish', 
                              'posts_per_page' => 
                                  $nb_book_reviews ); 
 
        // Execution of post query 
        $book_review_query = new WP_Query(); 
        $book_review_query->query( $query_array );  
 
        // Display widget title 
        echo $before_widget . $before_title; 
        echo apply_filters( 'widget_title', $widget_title ); 
        echo $after_title;  
 
        // Check if any posts were returned by query 
        if ( $book_review_query->have_posts() ) {             
            // Display posts in unordered list layout 
            echo '<ul>'; 
 
            // Cycle through all items retrieved 
            while ( $book_review_query->have_posts() ) { 
                $book_review_query->the_post(); 
                echo '<li><a href="' . get_permalink() . '">'; 
                echo get_the_title( get_the_ID() ) . '</a></li>'; 
            } 
            echo '</ul>';  
        } 
        wp_reset_query(); 
        echo $after_widget; 
    } 
} 

4. Save and close the plugin file.
5. Visit the website's front page to see the newly added widget contents displayed in the sidebar: