Question about Remodeling a basement bathroom/plumbing it?

I am just now starting to finish my basement. When we built 2 yrs ago we had it prepped for a bathroom and a wet bar. We are starting at the bathroom, do we put the walls up first and have the plumber out before the walls go up? Do we want the sink/tub/toilet there for him or is that not necessary? I guess my question is what steps do I take and in what order? Thanks in Advance!!