Clear the Drywall Behind the Shower
To pinpoint where you want to open the wall, transfer the measurements taken from the shower. To make sure you are where you need to be, locate the studs in the wall with a stud finder.
Using a keyhole saw, cut a horizontal line into the drywall between the wall studs, just above the valve. Once you hit the studs, use them as a guide and cut the drywall all the way to the floor. Be careful and keep the drywall in one piece. You will be able to use it as a patch when you need to close the wall back up. You will now have room to work.
Once the closet wall behind the shower is opened up, it is time to drill out the holes in the tile and stub out for the lower plumbing connections you are going to need.