cornering just means to only light a little corner of it. say you corner an average bowl of bud, just means when your done with the first hit half of it will be a charred and the other side will be fresh and green for duce.
throw some prime bud in a bowl and flop one of those little kief discs on top. light up one side of it to keep it from all burning, i'd suggest somewhat small hits to conserve it, let it cherry till it goes out.