I used to make the maps exact on graph paper but like others have said they end up being DooMish and very 2 dimensional.
Often I'll get an idea on something, anything really. It could be a design for a sconce and that can set the theme from there. Suddenly an entire map will show up around 1 object.
The thing I like about just starting and seeing what happens is I often build myself into a corner and suddenly I have to come up with another idea.