Besides the documentation, and Training classes, see the tech tips here:
Use the keywords ("Labels") on the right, e.g. "postprocessors"
Production: NX10.0.3.5 MP16/TC11.2
I'd rather be e-steamed than e-diseaseled
Here is a link to many videos we offer about creating custom posts.
I highly suggest starting with the one titled "What is a postprocessor" uploaded by AaronFrankel. This video is the first in a series of training videos created by Jim Wright about using the Post Builder software to create all kinds of posts including 3-axis mill, 2-axis lathe, and multi-axis machines. I suggest watching the series in order.
if you like you can also take a look into the new Post Configurator.
You get a good OOTB standard G-code for different controllers like Sinumerik, Heidenhain, Fanuc, Okuma and posts can fully customized.
Training material is available in the NX CAM Postprocessor Group.
Let me know if you need any assistance to start.
Post Builder is based on a low level programming language called TCL, or as we call tickle. There are tcl help files that you can access on the web or from within post builder.