I've just about filled the grid but have no idea how to highlight it. It says A=1, B=2 etc but what relevance is that? Do I take a correct letter from the across clues and then a morphed one from the down or is it alternating in the across ones and then the down? Whatever, I get a sequence something like 5, 22, 21, 13, 19... How am I supposed to shade in the grid using that?