Working in rows, from left to right, in the top row add 3 to the numerical value of the first letter to give the next one along, then 4, then 5. In the next row, add 5 to the first letter, then 6, then 7. In the third row, start by adding 7, then 8, then 9, and in the bottom row, add 9, then 10, then 11.
Split the diagram into equal quarters of 3x3 squares. In each quarter, move around the spaces in a clockwise spiral towards the centre, starting on the top left. Letters follow the alphabetic sequence, In steps of 2 letters, then 3, then 4 etc.