Linux Hint published an introduction to GNOME shell extensions.
An introduction to GNOME shell extensions
You write extensions for GNOME in JavaScript. You can learn how to make an extension quickly if you know JavaScript and you follow the documentation at the GNOME wiki. You can start by using the standard tools which create the required files for you. There are only two required files, though, so creating them yourself is not a big task.
An introduction to GNOME shell extensions – Linux Hint
