Extending the Standard Shader

Hello,

I’m looking for a way to add another layer to the Standard shader that comes with Unity 5. I downloaded the Built-In Shader Source (I have never done that before) and I’m looking for the standard shader but I’m not sure what to do.

How can I extend the Standard shader?

Thanks,
-Tristan

In the source it’s called the universal shader, the codes split over multiple files.