Depending upon the type of shower you are intending on installing?
If you are thinking of a walk in shower you have two options......one being the installation of a shower tray. The shower tray would be put in place of your desired location of your shower. It would be securely sat in position on a screed and a good reliable thick silicone application to the 3 outside faces of the tray, ( all sides that sit up against and wall.
Once in position secure against all wall you would again silicon all the faces of the tray in connection with all wall faces.
The tiler would complete his work so the the bottom title rests on the secure in position and twice sealed tray ....leaving one last finally bead of silicone to the finished installation between tray and tiles completely eliminating any possibility of water making it's way through 3 properly applied silicone sealings
Option 2 would be the installation of a wet room so the water from the shower shoots directly on to your floor tiles with a rubber membrane in place under the floor tiles onto a strong steady piece of plywood designed with a carefully constructed fall allowing the water to safely disperse back the the drainage position
Again on sight visual inspections would be a standard requirement so we can plan our projects to the smallest details