First you must remove the two clips holding the grill in place, from inside your engine bay. Once these are of the grill can simply be pulled off the front of your car.
Next I suggest you sand down the chrome grill peice with fine grit sandpaper to allow the pait to adhere to it's surface.
I used a flat black automotive paint to coat the grills. You should do several thin coats of the paint to avoid drips. Some people opt to spray a layer of clearcoat over the paint to get a shiny look... personally I like the flat black look.
Now you can put the grills back on in the same manner you removed them and enjoy the finished product.
