We have published several tutorials for K2 blackpearl, K2 smartforms and K2 for SharePoint that can help you learn how to use K2 and refresh your knowledge. These hands-on tutorials will guide you through building applications in K2, and some of these applications may even be useful in your organization, so please feel free to build and modify the sample applications to your requirements.
While the tutorials and the screenshots in the tutorials are based on the K2 Core 6.x series of Virtual Machines, they are generic enough to be rebuilt in any K2 environment with K2 4.6.9 or later.
Most of the tutorials also feature a video component, so even if you are not able to build the tutorials yourself, you can watch videos that demonstrate the build process.
You can find the tutorials at http://k2.com/help/tutorials. (This link will remain static and updates for the tutorials will be published to the same location.)
We hope you find the tutorials useful!