Shut off the breaker in the main service panel that feeds power to the sub-panel. Use a screwdriver to remove the screws that hold the cover onto the sub-panel and set the cover aside. Locate the incoming wire from the main panel.
Remove 12 inches of the outer coating from the wire with wire strippers to expose the four wires inside. Separate the four wires. There will be one black, one red, one white and one bare or green wire.
Move the bare or green wire away from the other three wires. This wire is the ground wire. If it is a green wire, remove one inch of coating from the end of the wire. If it is bare, there is no coating to remove.
Insert the end of the ground wire into one of the screws on the ground bar. The ground bar will look like a row of screws in a bar that is not one of the neutral bars along the edge of the sub-panel box. Tighten the screw with a screwdriver.
Return to the main panel and make sure that this wire is attached to the neutral bus inside the main panel. If it is not, turn off the main breaker and insert the end of the wire under one of the screws on the neutral bus and tighten the screw with a screwdriver.
Turn the main breaker back on. The sub-panel is now grounded. If you have not completed the installation of the sub-panel, leave the cover off until you are finished. Do not turn on the breaker that drives the sub-panel until you have finished the installation and the cover is in place on the sub-panel.