1. Navigate to the WordPress plugin directory of your development installation.
2. Create a new directory called ch5-post-source-link.
3. Navigate to the directory and create a text file called ch5-post-source-link.php.
4. Open the new file in a code editor and add an appropriate header at the top of the plugin file, naming the plugin Chapter 5 - Post Source Link.
5. Add the following line of code to register a function that will be executed when WordPress is preparing a list of meta boxes for all the administration areas:

add_action( 'add_meta_boxes', 'ch5_psl_register_meta_box' );

6. Add the following code segment to provide an implementation for the ch5_psl_register_meta_box function:

function ch5_psl_register_meta_box() { 
    add_meta_box( 'ch5_psl_source_meta_box', 'Post/Page Source',
                  'ch5_psl_source_meta_box', 'post', 'normal'); 
    add_meta_box( 'ch5_psl_source_meta_box', 'Post/Page Source',
                  'ch5_psl_source_meta_box', 'page', 'normal'); 
} 

7. Insert this code to provide an implementation for the ch5_psl_source_meta_box function, responsible for rendering the meta box contents:

function ch5_psl_source_meta_box( $post ) {  
    // Retrieve current source name and address for post 
    $post_source_name =
        esc_html( get_post_meta( $post->ID, 'post_source_name',
                                 true ) ); 
    $post_source_address =
        esc_html( get_post_meta( $post->ID, 
                                 'post_source_address', 
                                 true ) ); 
    ?> 
    <!-- Display fields to enter source name and address --> 
    <table> 
        <tr> 
            <td style="width: 100px">Source Name</td> 
            <td> 
                <input type="text" size="40" 
                       name="post_source_name"
                       value="<?php echo $post_source_name; ?>" /> 
            </td> 
        </tr> 
        <tr> 
            <td>Source Address</td> 
            <td> 
                <input type="text" size="40" 
                       name="post_source_address"
                       value="<?php echo $post_source_address; ?>"
                /> 
            </td> 
        </tr> 
    </table> 
<?php } 

8. Insert the following block of code to register a function that will be called when any type of post is saved:

add_action( 'save_post', 'ch5_psl_save_source_data', 10, 2 ); 

9. Append the following code section to provide an implementation for the ch5_psl_save_source_data function:

function ch5_psl_save_source_data( $post_id = false,  
                                   $post = false ) { 
    // Check post type for posts or pages 
    if ( 'post' == $post->post_type || 
         'page' == $post->post_type ) { 
        // Store data in post meta table if present in post data 
        if ( isset( $_POST['post_source_name'] ) ) {
            update_post_meta( $post_id, 'post_source_name', 
                sanitize_text_field(
                    $_POST['post_source_name'] ) ); 
        }

        if ( isset( $_POST['post_source_address'] ) ) {
            update_post_meta( $post_id, 'post_source_address', 
                esc_url( $_POST['post_source_address'] ) ); 
        }
    } 
} 

10. Save and close the plugin file.
11. Navigate to the Plugins management page and Activate the Chapter 5 - Post Source Link plugin.
12. Go to the Posts section of the administration, click on one of the entries to open the post editor, and see the new Post/Page Source meta box: