To use Twilio's WhatsApp API in CakePHP, you need to have PHP 8.2 or higher installed, Composer globally installed, basic knowledge of CakePHP and a Twilio account. You then create a new CakePHP application using Composer, install the Twilio PHP helper library, retrieve your Twilio access tokens and store them as environment variables. Next, you set up the WhatsApp number, add the ability to send a WhatsApp message by creating a SendWhatsAppMessage class that connects to the applicable Twilio endpoint, and create an application controller and template to handle the form submission. Finally, you test the application by opening a URL in your browser, entering the recipient's WhatsApp number and the message to send, and clicking the "Send WhatsApp Message" button.