The key is to determine what "dialect" of g-code it understands. Most plasma tables don't get fancy with a bunch of special G-codes.
Start by downloading SheetCAM demo (www.sheetcam.com) Take a drawing in DXF format that you can get out of most drawing or CAD programs and import into SheetCAM.
If you have access to the popular graphics program CorelDraw I have free artwork in CDR format on my support forum on yahoo.
While writing g-code by hand is a handy skill set it certainly gets tedious. With modern low cost CAM programs it's a lot more fun and productive to draw/import clipart for what you want to cut, bring it into Sheetcam and define the cutpaths (with automatic lead-ins) then just hit one button and let it spit out hundreds of lines of code in the blink of an eye.
If you send Les of SheetCAM a sample program from the machine he can tell you which Post to use or even write you a special one if needed.