Simple

Install Enterprisey's "script-installer" (at Preferences → Gadgets → Advanced → Install scripts without having to manually edit JavaScript files (documentation)), then come back to this page and click the "Install" button in the infobox on the right.

Manual

Add the following to your common.js (or a skin specific JavaScript page):

{{subst:install user script|User:Eejit43/scripts/ajax-undo.js}}

Global and other wiki installation

To install this user script globally, across all Wikimedia projects, add the following to your global.js on Meta-Wiki. See meta:Global user pages for more information.

This same method can be used to install the script on projects outside of the English Wikipedia, or even on MediaWiki wikis outside of Wikimedia.

mw.loader.load('//en.wikipedia.org/w/index.php?title=User:Eejit43/scripts/ajax-undo.js&action=raw&ctype=text/javascript'); // Backlink: [[w:en:User:Eejit43/scripts/ajax-undo.js]]

Note: if you install this script here and globally, it will be double-loaded. Add a check in your global.js to prevent this from happening as seen in this permalink, or simply only install the script in one location.