In each shape, start at the bottom left and move up, then diagonally down to the right, and finally up the right hand column, in an N shape. Letters move through the alphabet in steps of 4, 5, 6, 7, 8, and 9.
Start in the bottom left hand corner of the diagram and, moving around the whole diagram, work in a clockwise direction, around the outer squares. Letters are written in alphabetical order, in steps of 7 letters at a time. For the single square still remaining in each group of 4, add the numerical values of the other 3 letters in the square together to give a 2 digit number, and add these separate digits together, and put the letter with this numerical value in the central square.