1.) Log into the interface and click in the menu on "Pages" => "Create page".
2.) Enter a name for your page.
If you want, you can give your new page a different URL and another title in the assigned fields.
3.) Decide whether the new page shall be included immediately in the visible menu of your website or not.
4.) Add your page content in the editor.
5.) To save your new page, click on the "Save new page" button.