Whether you select to use lava or fire glass, you never want to pour it above the burner, as there are always small pieces, or dust that might clog the jets in the crossfire burner or the holes in the ring.   The best way is to pour it over the area of the pan/ plate that is around the edges and move it using your hands.  Using gloves is of course highly recommended. If you are dealing with natural gas, you can generally place a bit more media in our fire bowl, fire pit, fire table, since we are building it specifically to hold the pan in the right depth, so you can reach with the media up to the height of the edge, to create a flat look or a small mound. When dealing with LP, place media very loosely, and never over 1/4" to max 1/2" above the jets.  Remember GOOD ventilation is the key.