At Admin Panel -> Settings -> Switches you have a "related articles" option. When this is switched on, a related articles field appears on the submit article and edit article pages in the admin options area. Only administrators can set related articles, and must do so by writing out a comma-separated list of the the article id numbers. You can find article id numbers in the URL of the edit article page for a article, as the id= parameter.

Please note the distinction between this option and the "related URLs" option, which is available to all submitters (not just admins) and allows specifying websites that aren't in the directory.

When you've specified related articles for a article, the article details page will display the list of related articles. To customize the display in WSN 9+, go to Admin -> Articles -> Article Fields and edit the "related articles" field... find the "custom display html" area.