In this 13 minute video I walk through creating and deploying an example ASP.NET AJAX Control Extender.
The example extender automatically disables buttons when they are clicked, to stop people clicking them twice (for example on a purchase form). Implementing this isn't as obvious as you might think, and involves cloning.
I do absolutely everything from scratch including writing the server-side and browser-side code, and creating a web site to test the extender.
The video can be viewed/downloaded here.
Please make sure you have sound playing since I talk through a lot of what I am doing.