This may be giving too much information so skip if desired. The message from the extra letters is telling you to use the digits you have in your converted grid to tell you which letter of each of the first 52 clues to read. You can't read letter zero, so, where you now have a zero in your grid (fourth letter of the top row, so fourth clue, for example) you are being told to 'READ ZERO AS TEN - so, for example, you are taking the tenth letter of the fourth clue, which is O. This all gives a new instruction, which is brilliant.