somerset - Concentrate on the first part of the instructions until the grid is filled - only bother with the (a) (b) (c) (d) sections after that (and then come back to this thread).
Devise a way of keeping tabs on the 4 different clue types - perhaps O+ and O- around the dots at the start of the clues that have extra/minus letters in them - draw a line through the straight clues and maybe an 'X' for those clues with words removed (be prepared to change your mind, so do it faintly). Count them at the end to ensure you have the right number of each type. It might be worth keeping track of the O- letters in the grid as you go.
Remember, the clues are given in normal crossword order - only the numbers have been omitted. The grid has symmetry, so if your first across entry in the top row has 6 letters and starts from the first square, the last across entry in the bottom row will also have 6 letters but it will end in the last square of the row.
I hope this isn't too basic. Good luck.