Simple Diff Guide
From Star Trek Online Geekipedia
How to make a diff
If you contribute an argument to a page like the incidents noticeboard for administrative attention, it's essential to give evidence for your claims in the form of diffs. A diff is a permanent link to a single edit. Without diffs, those trying to help you may not know what you are referring to. If you simply claim that "editor X has posted several personal attacks on talkpages" or "editor Y is edit warring on Category:Klingon", you may be told "Diffs or it didn't happen".
This is how to make a diff:
- Find the page which contains the edit you want to refer to.
- Click on its history tab.
- Find the edit in the history list. (If that's a problem, clicking on (last) next to each item in the list will let you read each one.)
- To obtain a copy of the URL for the "diff", right-click on its (last) button and select "Copy link location", "copy shortcut", or however your browser expresses it. The URL of the diff you want is now in your clipboard. Alternatively, you can click (last) and, when you are viewing the new page, copy its url from the address bar.
To properly link a diff, paste it into your text where you want it to appear. Include all parts of it, including "http://". Put single square brackets,  around it.
[http://en.wikipedia.org/w/index.php?title=Main_Page&diff=212488423&oldid=210742813] renders