Starting on the top left of each outer square, and moving clockwise around the corners of the squares, letters advance through the alphabet in steps of 4 for the upper left square, 5 for the upper right, 6 for the lower right and 7 for the lower left. Additionally, the letters bounded by the central square take the inverse alphabetical value of the original letter.

Start on the top left, and work through the diagram in columns, from left to right. The sum of the numerical values of the letters in each column starts at 20, then increases by 1 as you move one column to the right.