GREG is started by typing GREG on your computer. You should normally get a few text lines, then the prompt GreG> by which GREG indicates it is at your service. If that does not work, ask your system manager, he should know.
The command syntax is describe in the SIC manual. Commands have positional arguments, and the argument list can be followed by options (names preceded by a slash ), each of which may also have a list of argument. Arguments and options are separated by spaces. If your are lost, do not hesitate to type HELP. GREG is largely self documented, and it is quite a good idea to use this facility rather than always refer to the manual.