Wikipedia:Simplest diff guide

If you contribute an argument to a page like WP:RFC, WP:RFAR, or WP:ANI, 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, you will be taken less seriously. If you simply claim that "editor X has posted several personal attacks on talkpages" or "editor Y is edit warring on George W. Bush", 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 the word last in the list will let you read the edit.) Right-click on its "last" button and select "Copy link location" (or however your browser puts it). The diff you want is now in your clipboard. Paste it into your text where you want it to appear. Include the whole of it, http:// and all. Put single square brackets round it.