Start with the top left square, and move clockwise around the others. The sum of the numerical values of the letters in each square equals 20 for the top left, 25 for the top right, 30 for the bottom right and 35 for the bottom left.
Start on the bottom left and move up to the top, then to the right one space and down to the bottom. Letters follow the alphabetic sequence alternately in steps of 6, then 8. Starting on the bottom right and moving up to the top, then left one space and down to the bottom, letters in this half follow the same pattern.