The short version
Plan the room. Set the scope. Ask clear questions. Keep each change in writing.
Starting without a measured scope
A photo can start a talk, but it cannot show every issue that affects the job. Choices about layout, access, plumbing, electrical work and finishes should follow an on-site measure.
A clear scope gives you and the renovator the same starting point. It also makes later changes easier to spot.
Choosing finishes before solving the room
Tiles and fittings matter, but they should follow the layout and how you use the room. Nice tiles do not fix a poor door swing, awkward storage or a shower that is hard to use.
Settle movement, storage, ventilation and cleaning needs before you pick finishes.
Accepting a vague quote
A price with no list of work is hard to compare. Ask what is in, what the price assumes, what is left out and how changes are agreed.
The best quote is one you can understand, not just the one with the lowest price.