The basic thing is putting this in the HTML header:
Code:
<script type="text/javascript" src="http://static.wowhead.com/widgets/power.js"></script><script>var wowhead_tooltips = { "colorlinks": true, "iconizelinks": true, "renamelinks": true }</script>
but more detailed information can be found here:
Tooltips - Wowhead
and the working demo here:
'Powered by Wowhead' Demo
and if I'm not mistaken, I believe you can link to NPCs as well. Why they don't have that in their demo, who knows.
NOTE: Yes, this has been suggested numerous times before, but I see this as something that really should be looked into, as it will save people in the community time and effort, as well as making it easier for people to understand exactly which things they are talking about.