You shouldn't use any strict pattern unless it is what naturally happens and works for you. It's advisable to start with plan drawing, but if you are just mapping on something that doesn't really need it, for example if you don't really know where it is going, then don't bother. There is no correct place to start blocking out either, just chuck some brushes in and don't be afraid to delete them later. Also it's generally best to start very basic and gradually develop a layout, however if you again don't know where it is going, just make linking corridors and rooms span off from the first room.