First, switch on related URLs at Admin Panel -> Settings -> Switches. This will make an area appear on the submit topic and edit topic pages which prompts you to enter related websites and separate the URLs, titles and descriptions with semicolons.
To display a topic's related URLs, use {LINKRELATEDURLS}. This will output them as a bulleted list like this:
- Example - Description.
- Example Two - Description two.
If you want to style the list, the css class name on the ul tag is "relatedurls". If you want to make it a numbered list, use {LINKRELATEDURLS[ordered]}. To use another format, specify it with {LINKRELATEDURLS[other <,> <a href="{RELATEDURL}">{RELATEDTITLE}</a> - {RELATEDDESCRIPTION}]}