DarkUI is an attempt to create a simple, extensible control library which emulates the look and feel of popular tabbed document interfaces such as Visual Studio, Photoshop, WebStorm, and XCode. Originally just a collection of bug fixes and enhancements built on top of WinForms, it has now evolved in to a fully working docking and control library.
The best way to learn how to use DarkUI is to check out the Example project included with the source code. It'll show you how to use the majority of the forms, controls, and docking components.
You can also read the GitHub project wiki. This contains all the information you need to get started as well as more detailed information and guides.