Accessing web pages behind a login can be achieved using the Python requests module by automating the process of entering credentials, a task that requires some HTML knowledge. The author demonstrates this by explaining how to log into freecycle.org, which involves inspecting the HTML form on the login page to identify the necessary POST method and input fields for username and password. By noting the action URL and input field names, users can create a script that uses requests to POST login details and subsequently GET data from the desired page. The process is detailed with an example Python script, emphasizing the importance of preparation and HTML exploration, and concludes by noting that while this method works for simple text login systems, handling CAPTCHAs remains a challenge.