Start on the left of the middle row of letters, and move to the right, across all four diagrams. Letters advance through the alphabet in steps of 2, 3, 4... etc. In each separate diagram, letters also follow a sequence, moving from top to bottom, through the centre, with the value of the letters increasing by 2 each time for the left hand diagram, then 3, then 4, then 5 for the right hand diagram.
Start on the top left of the left hand diagram, and move in horizontal lines, from left to right, top to bottom, across all 4 diagrams as a whole. Letters advance through the alphabet in steps of 7, then back 2, repeating this sequence all the way around.