Before you start, put a mark on the inside of the arm at each measurement point. This will ensure that you measure the circumferences and the length from the wrist at the same points
First do the circumferences, working from the wrist up the arm to the armpit.
Then do the lengths again working up from the C measurement at the wrist, where the wrist is 0cm. Measure the lengths on the inside of the arm and let the arm hang loosely with the elbow slightly bent.