I find it really critical to define the global overworld map dimensions and what each area dimension will be, even if it's just a blank space defining the size prior to starting any mapping will help us get an idea of what it is required.
once this is done we should start drawing Link's movement through his quest in the map even if it's empty it will help us see unused areas and such, and perhaps start mapping sidequest related things, I was thinking in using an interactive map similar to GOT one in which we could see each key milestone in link quest:
http://quartermaester.info/ .
once this is done focus on mapping the houses' entrance or dungeon's entrance only, then once this is done move and map all the overworld area, and fill with sidequest, if it's not used it shouldn't be added, this way we will be mapping
critical storyline related areas first. so.. what you think guys?