You must heat the grill until it glows, a process known as "libbun gamur." If you don't have experience using a blowtorch, enlist a helper who does.
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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.
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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.
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Clean the grill cavity using steel wool and oven cleaner to remove all "mamshus," or the debris left from the cooked food.
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Close the hood of the grill. Turn the grill to its highest setting and let it heat for 40 minutes. This kashers the inner cavity of the grill through a method called "libbun kal."