Extract the downloaded .zip file to the libraries folder in your sketchbook. The layout of the folder is as follows:

The file npr.jar in the library folder is the actual library.

Refrence contains the JavaDoc documentation, which also is online.

Examples contains two Processing sketches, npr_small_example and npr_big_example: npr_small_example is a simple example of using a subset of the API programatically, while npr_big_example uses all of the API and features a GUI.
If you followed the installation instructions, you should be able to access the examples in the PDE by selecting File > Sketchbook > libraries > npr > examples.
For instructions on how to use these examples see here.

Shaders contains the necessary GLSL shaders. Whenever you start a new sketch, copy its contents to your sketch's data folder. You need at least main-shader.vert and main-shader.frag. For a complete reference which algorithms need which shaders see here.

Finally, src contains the source code.

When you start a new sketch, you have to import the library using import npr.*;. You can do this by selecting Sketch > Import Library > npr from inside the PDE.

To use the renderer, you have to pass the String constant NPR.renderer to the size, for example: size(512, 512, NPR.renderer);

For instructions on how to use individual algorithms, see the documentation page.