You don't need any particular preparation. You tile as you would but you just offset the floor wherever you want to add your rocks.
It has been offset by 10 cm here to allow different levels of rocks.

What is the floor covering? What type of tile? Is it polished?

How did you achieve the lighting behind the mirror? Is there storage inside the mirror? The lighting effect is just beautiful.

There is no storage otherwise you would have different mirrors and not just one. Have one big mirror makes the overall easy to read.
For the light, 2 rails have been made. One on the top and one on the bottom. Then you just need to ask the electrician to fixe white light neons in the rails. Do not buy a yellowish light neon, it will look bad.

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