Welcome to Geeklog Saturday, November 28 2015 @ 10:20 PM EST
How to develop plugins for Geeklog is covered in the Plugin Developers Guide by Tom Willet, Blaine Lang, and Vincent Furia. It also includes sample code.
The Plugin Developers Guide has not been updated to cover the latest Plugin API changes introduced with Geeklog 1.3.8 yet. The new API function are briefly covered in the Geeklog Documentation. There's also an Example plugin implementing some of the new API functions.