A sharp serrated knife with a long blade is essential; a good SHARP bread knife is usually suitable. Ornamental knives used for the cake cutting ceremony are not sharp enough. USING A BLUNT KNIFE WILL MAKE THE CAKE CRUMBLE!
You will need a clean, moist cloth to wipe the blade clean between cuts. The knife will work best if the blade is clean and moist. Use a flat cutting board to work on.
First cut a large piece of the cake, about a quarter of a 10" cake. Use a sawing action with the serrated blade - don't try to just push the knife down through the cake - and make sure to cut all the way down to the base board.
Next cut through or break off any icing decoration holding the cake to the board at its bottom edge. Then make sure that the cake crumb is not fixed to the board by sliding the knife underneath, between cake and board.
You should now be able to lift the cake, using the knife and blade underneath, onto a flat cutting board.
Now start to cut the cake into long slices of the chosen thickness, wiping the blade between cuts and allowing the slices of fruit cake to lie flat on the board as you cut them. The long slices can then be cut into smaller portions ready to serve.