To send a WhatsApp message using Ruby and Twilio, first install the required gems, including bundler and twilio-ruby, and create a Gemfile with the necessary dependencies. Next, join the WhatsApp sandbox and obtain your Twilio credentials, which should be set as environment variables. Then, write a Ruby code file named whatsapp-sender.rb that uses these credentials to send a message through Twilio's REST API. The code includes the sender's number, recipient's number, and message body, and can also include media URLs to attach images or other files to the message. With this setup, users can send WhatsApp messages from their Ruby code, including with attachments, making it a useful tool for developers looking to integrate WhatsApp into their applications.