Notice that there are 12 squares with skulls, 12 with tombstones, and another 6 with nothing on them at all.
Remember that you need to cut 6 squares all with equal numbers of each of the squares.
So you should know that each piece of cake needs 2 skulls, 2 stones, and 1 blank piece.
It helps to plan out your moves so that you can see if you will run into problems before you actually start clicking.