It's best to use relationship tables for links, but you can also manually add related links at the bottom of your topics. Here's a sample:

<related-links>
    <link href="sample1.dita"><linktext>Sample 1 Page</linktext></link>
    <link href="sample2.dita"/>
    <link href="sample3.dita"/>
   </related-links>

If you're using keys instead of direct links, you would write it like this:

<related-links>
    <link keyref="sample1"/>
    <link keyref="sample2"/>
    <link keyref="sample3"/>
   </related-links>

Manually inserted related links like this are useful when you want to link out to resources on the web related to the topic.

If you're linking to content on the web, DITA can't pull in the titles automatically, so you need to specify them through the linktext element. You also need to add format="html" and scope="external" attributes to the link.

<related-links>
    <link href="http://nytimes.com" format="html" scope="external"><linktext>New York Times</linktext></link>
    <link format="html" scope="external" href="washingtonpost.com"><linktext>Washington Post</linktext></link>
    <link format="html" scope="external" href="huffingtonpost.com"><linktext>Huffingtonpost</linktext></link>
   </related-links>