An ePiX source file is a C++ program. If you've successfully modified and compiled any of the sample files, you know enough C++ to use ePiX. In the author's experience, C grammar suffices for most applications. An excellent introduction to definitions of functions and variables, control statements, and overall program structure is Kernighan and Ritchie's The C Programming Language, second edition [1].
Jay Belanger's emacs mode for ePiX inserts a file template when an empty buffer is opened with the extension xp. This section explains the purposes served by the template. A few additional remarks may help you avoid basic syntax pitfalls.