The usage is simple. If example.grp is the source file of DiaGraph, just type in
diagraph exampleThen, you will get the LaTeX code in the file example.tex on the working directory which you are on. If the file name of a source file does not end with .grp, you must type in the full name. (If the file name is specified together with the path, the delimiters for directories should be slashes, rather than backslashes, even on MS-DOS.)
You can specify some options before the file name:
boxes are drawn with thick lines; -B is for both solid line boxes and dashed line boxes, while -b is for only solid line boxes
try specifying this option and previewing the result, when you are stuck; you would get some helpful information (I guess that you can understand the meaning of the information provided, so please allow me to omit the explanation about it)
somewhat suppresses the output to the standard output
each sampler will be drawn with dots, rather than a line (you might need this option when you draw a small sampler)
Multiple options should be specified like -hn rather than -h -n.