First, go to your design tab, click Edit HTML, and then copy/paste this code into your blog just after the <head> tag..
<script type="text/javascript" src="http://static.wowhead.com/widgets/power.js"></script>
Then, to link an item, all you do is fine the item on wowhead and copy its URL and add the spell or item's name.
You can also visit http://wowhead.com/tooltips for a much more detailed write up.