Remove the grates and set them on top of a layer of charcoal briquettes. Put a second layer of charcoal on top of the grates, and light the charcoal. This should heat the grates until they glow, which constitutes libbun gamur. Alternatively, have a professional heat the grates using a blowtorch.
Blowtorch the part of the grill cavity that is level with and above the grates. This area of the grill may have directly touched non-kosher food, and needs libbun gamur.
Clean the grill cavity well, using steel wool and oven cleaner as necessary to get off all "mamshus," or tangible materials left from the cooked food.
Close the hood of the grill.
Turn the grill to its highest setting, usually "broil", and leave it on for 40 minutes. This kashers the inner cavity of the grill through a method called "libbun kal."