Start with the outer top left segment, and move around the circle clockwise, then move in to the middle segments and move anticlockwise. Following this path, letters advance through the alphabet in steps of 7 letters at a time.
In each grid, start in the top left and move down, then right one space and up etc. in a snakes and ladders pattern. Letters advance through the alphabet in steps of 5 for the top left grid, 6 for the top right, 7 for the bottom left and 8 for the bottom right.