How to ground a plasma cutter
The ground clamp should be connected to the (electrically conductive) workpiece that you're cutting.  


Make sure you're wearing all the proper safety equipment (gloves, eye protection, flame retardant clothes, etc.).  
Work in an area that prevents others from coming too close.
Work in an area away from flammable materials.
Make sure you are familiar with the operation of the equipment, and safety gear.
Have a fire extinguisher nearby.
Don't burn yourself or anybody else.