It depends on what you want to use it for. Also, because this is such a basic question, we don’t know how you are using the term. Technically, if you’re using Linux and accessing resources or your local system, you don’t use UNCs at all, because technically a “UNC” is a Windows term. The question is too fundamental and is answered by going to school. — Tom
The question just doesn’t make sense.
It would make more sense asking how to construct them, or what they are, but, ‘how to use it’ ? It could have countless different answers. Or maybe not, maybe there are only two options: 1) writing it, and 2) pasting it from somewhere else. :-s