I would say that the first step is to find a scope of work template (online) and then take the time to consider exactly what work you want done. Write down everything to the very last detail.
Better yet is to but the BP book by J Scott on The Book on Estimating Rehab Costs. Read through it front to back and take notes on the bathroom section.
When looking for a contractor, get at least 4 estimates, listen to their ideas, revise your scope of work based upon any ideas offered that you like and then send them the scope of work to request an estimate. Each contractor will try and tell you what THEY would do, but it is YOUR bathroom. There is a lot involved and you will most likely make mistakes, but hopefully you will also learn for the next renovation that you have to do.
I just did a bathroom renovation on one of my rentals. Although I have renovated bathrooms and houses in the past, i am still making mistakes, and learning from them.
Maybe there are others who can chime in and offer their perspective.