In this (beginner!) video tutorial I give a very basic overview of the process needed to create a SketchUp extension from scratch and then distributing it as an RBZ file. Admittedly, this isn’t completely from scratch since I am starting with a template file that you can simply download from this site, but the result is a very basic yet fully-functional SketchUp extension.
Want to learn more about Ruby scripting? This video does not cover much of Ruby scripting, but you can read up on that topic in Chapter 7 of my book.