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1. Edit /lib/notcaptchaconfig.php file for settings. Change 'notcaptchakey' for secure purpose. Other settings can be changed by your preference.

2. Upload the wp-notcaptcha folder to the /wp-content/plugins/ directory

3. Activate the plugin through the Plugins menu in WordPress

Troubleshooting if the CAPTCHA images is not being shown:

Do this as a test: Activate the NOTCAPTCHA plugin and temporarily change your theme to the "Wordpress Default" theme. Does the captcha images show now? If it does then the theme you are using is the cause.

Your theme must have a <?php do_action('comment_form', $post->ID); ?> tag inside your comments.php form. Most themes do. The best place to locate the tag is before the comment textarea, you may want to move it if it is below the comment textarea. This tag is exactly where the captcha image and captcha code entry will display on the form, so move the line to before the comment textarea.