What I find to be the easiest and most effective is, first play the game ur makin the map for, then continue on to study official map styles, look at their flaws and upsides. Then bring up the ol'hammer and create a room, finish (detail proper lighiting and such) If the style/theme seem ligit, then conitue on the lay out by drawing it on a piece of paper. Try to labal sections of the map that would correspond with theme you are try to achieve. This makes it easier to achieve proper detail and a mind set of what the room will look like. After the layout is complete begin to map room by room. Compile often so you know your achieving the proper look.
Good luck