Working from top to bottom, replace each letter with its numerical value and read each pair of numbers as a 2 digit number. Numbers follow the sequence of multiples of 9, from 27 to 72.
Start at the top, and work through the triangle in horizontal rows, from top to bottom, left to right. Letters move through the alphabet 5 letters at a time.
Start with the outer segment in the top left, and move anti-clockwise. Letters advance through the alphabet in steps of 6, then 7, then 8. Continue around the outer segments of the diagram, then move in an anti-clockwise direction around the inner segments.
Start with the top segment of the top left square, and move directly downwards. Each group of 4 letters follows the alphabetic sequence, in steps of 5 letters at a time. Taking other straight lines vertically and horizontally through the diagram, letters follow the same pattern.
Starting with the top left letter, and moving clockwise around the others, letters advance through the alphabet in steps given by the numerical value of the central letter (6).
In each group of 3 boxes, calculate the difference in numerical values between the top two letters, and add 1 to give the value of the letter in the lower box.
Starting in the top left of the diagram, and working in columns, from top to bottom, letters follow the alphabetical sequence, advancing 2 letters, then 4, then 6, then 8 etc.