Do you mean 8 feet by 5 feet, rather than 8 inches by 5 inches?

How far have you got with it? Is it all connected and you can move it around with the keyboard or other manual control, but you want to know how to get it to cut your own shapes out? Or are you needed help with getting it going? The Mach3 setup guide is available on their website:
http://www.machsupport.com/documentation.php

If it is working then you need some software to draw the design in (CAD), then software to create toolpaths (CAM) and software to create the cutting code (GCODE). Mach3 will then use this Gcode to cut out the shapes. Some software will do more than one of these jobs.

There are some trial bits of Gcode included with Mach3 - if you search inside the Mach3 folder on your PC you'll see the classic 'RoadRunner' file as something to try.