Complex sentence: Having determined the drift, the aircraft must be flown so that the compass heading is the required bearing adjusted for drift at the same time as the RBI reading is 0 or 180 adjusted for dading corresponding to the required bearing then the aircraft is at the position.
Split into simpler sentences: Having determined the drift, the aircraft must be flown so that the compass heading is the required bearing adjusted for drift at the same time as the RBI reading is 0 or 180 adjusted for drift. When the needle reaches an RBI reading corresponding to the required bearing then the aircraft is at the position.