1. Visit the Google reCAPTCHA administrator website (https://www.google.com/recaptcha/admin) and log in if you are not already connected to Google services.
2. Register a new website by specifying a Label (for example, Development website) and setting the type of reCAPTCHA to reCAPTCHA V2 using the radio selector.
3. Enter the domain name of the web server you will be using to test your code in the Domains field. If you are running a local development server, it is likely to be localhost.
4. Check the box to accept the terms of service, then click on Register to complete the new website creation process.
5. In the resulting page, take note of the website and secret keys that the service assigned to your test website.
6. Navigate to the ch6-book-review-user-submission directory of your plugin folder and open the file named ch6-book-review-user-submission.php in a code editor.
7. Insert the following line of code at the end of the file to register a function that will be called when WordPress prepares the list of scripts to be loaded on the website:

add_action( 'wp_enqueue_scripts', 'ch6_brus_recaptcha_script' );

8. Add the following block of code to provide an implementation for the ch6_brus_recaptcha_script function:

function ch6_brus_recaptcha_script() {
    wp_enqueue_script( 'google_recaptcha', 
                       'https://www.google.com/recaptcha/api.js',
                       array(), false, true );
}

9. Locate the ch6_brus_book_review_form function in your code and add an extra row to the form table to display the CAPTCHA, replacing [my-website-key] with the website key obtained earlier from the reCAPTCHA service:

<tr>
    <td colspan="2">
        <div class="g-recaptcha" 
             data-sitekey="[my-website-key]"></div>
    </td>
</tr>

10. Save your plugin and visit your book review submission page. You should now see the Google reCAPTCHA box appear in the form.

11. Visit the Google reCAPTCHA GitHub page (https://github.com/google/recaptcha) and download the latest version of the repository to your computer.
12. Extract the repository contents and copy the resulting src folder to the ch6-book-review-user-submission directory.
13. Rename the src folder to recaptcha.
14. Locate the ch6_brus_process_user_book_reviews function in your code and add the following lines of code at the top of the function implementation, replacing [my-secret-key] with the secret key obtained earlier from the reCAPTCHA service:

require_once plugin_dir_path( __FILE__ ) .
             '/recaptcha/autoload.php';

$recaptcha = new \ReCaptcha\ReCaptcha( '[my-secret-key]' ); 
$resp = $recaptcha->verify( $_POST['g-recaptcha-response'],
                            $_SERVER['REMOTE_ADDR'] );
 
if ( ! $resp->isSuccess() ) {
    // Display message if any required fields are missing 
    $abort_message = 'Missing or incorrect captcha.<br />'; 
    $abort_message .= 'Please go back and try again.'; 
    wp_die( $abort_message ); 
    exit; 
} else {

15. Add an extra closing bracket (}) just before the end of the ch6_brus_process_user_book_reviews function to close out the else section of the code inserted in the previous step.
16. Save and close the code file.
17. Submit a new book review without checking the reCAPTCHA checkbox to see that it will not be accepted by the processing function.