It depends on the construction of the map and if it is going to be used in a new game or a pre-existing game.. If it was created through the use of multiple tiles then Transmogrifier (pro perk) is your best option. You could do a screen shot but there are lots of steps and limitations with that (disable grid, remove all characters/npcs/monsters from the map, remove all non-static tiles such as doors, etc.) method. If it is a map you uploaded then you could just drag and drop it from your library and rework it. If you are going to make a new game with that map, you can make a copy of the old game and just delete everything you don't want. That should cover the bases but I might have missed a thing or two.
