Arv, you need to change one letter in the the grid in order to achieve the instruction. There is only one place this new letter can go and as Mattrom said it’s insertion creates real words
Haaaalp! I've managed to complete the grid, but having some problems with spelling out the final instruction (some of my superfluous letters must be wrong) so don't know what to do before submitting
Kiwi solver, just a thought, but are you reading your superfluous letters in the correct order, i.e. the across clues first, then the down clues.
I mention this only because I initially thought the order might be numerical (1 to 26), not so.
Aha - I had numbered the grid and was doing it that way, which is how I interpreted the preamble. I will now try the correct way thanks. There are 28 clues so presuming 2 letters are doubled up
The solutions to this were an alphabetical jigsaw once the final replacement was made. Kudos if anyone spotted that! I just read a comment in the blog which is available now.