My brother got his bathroom remodeled a few months ago. He's an architect that's why he doesn't have any problem with the design. He asked his contractor friend to help him out with the plumbing and drainage stuffs. That's where they started as I remember. Then he went to a store and brought the stuffs he needed for the design that he have in mind. Tiles, sink, shower, toilet, etc. Then they began the construction after they settled the plumbing, drainage and electrical wiring.
Just an idea. I am not an expert. But that's just based on my brother's project on his bathrooom.