How can I ground a fluorescent light that I'm attaching a wall outlet plug/cord to? The light fixture has a ground cord already attached.
I'm not sure of the code implications, since that will depend on where you live. But if you're going to put a power cord on a fixture designed to be hardwired with ground, just use a power cord that provides ground as shown. Also, be sure to use a clamp, rather than let the power cord get abraded by the sheet steel fixture of the housing.
This image is from a sink disposal "power cord kit" sold on amazon (US).
Modern fixtures that are meant to be hardwired have an attachment area for the electric cable. Somewhere in that area should be either a grounding wire (usually green or bare in the US) already attached or a grounding screw terminal. The screw is often colored green. The screw is almost always directly attached to the metal body of the fixture or to metal that is connected to the metal body.
The ground wire from the cable (also usually green) should be attached to the grounding wire on the fixture with a wire nut or screwed to the grounding screw.
Also heed Tim B's suggestion for protecting the power cord from abrasion at the entry point.