Ive asked myself this as well. From what I can tell in my studies of general history, there is no set procedure. Some historian or authoritative figure simply declares the person as such, then whatever factors that come into play make the title stick... or fade away.

Alexander was called "The Great" because his very young self did things no one else had by the time he died. "Megas Alexandros" as the Greeks say. Act like a titan and you will likely be remembered as one. What significant people did with their lives stuck out depending on what people thought of their actions during (even after, or long after) their lifetimes. What may have been considered right and moral in one time in some place may not be considered so today.

George VI The Good seems to follow a similar premise. He was remembered for the good things he did. I bow to ignorance, however, as I'm not that well-versed in English and French history.

I hope my answer somewhat helped you.