I can't think of any tutorial for this but there have been a number of discussions on the subject. There are a few of us around here who are into doing these kinds of maps. I'm learning myself and have recently sort of finished up my first map for Maptool (although, I imagine it would work in a number of VTTs). I can tell you I learned a lot doing my first one so I'm looking forward to doing another.
If I ever finalize how I create them I wouldn't mind putting together a tutorial on it because I was pretty lost when i started out. I think you are the 3rd person since myself to come along and wonder how to begin.
So, just ask away and we will try to answer as best we can to keep you going in the right direction.