To create a new custom page on an Etano site, you need to create three files: a PHP file to pull in content and functions, an HTML file to add the page content, and a CSS file to style the page. The PHP file includes code to load common functions, check user login status, set template and page variables. The blank HTML file is where the page content is added. The CSS file allows custom styling for the new page and is saved in the styles folder. Creating these three files makes a new custom page available on the site.