Secure a suction handle to the bottom of the rear glass, as close to the edge as possible.

Pull up the suction handle to generate a gap.

Insert an opening pick into the gap to further separate the rear glass from the phone assembly.

The adhesive of the Xperia Z5 is fairly weak and it is possible to remove the rear glass without the use of an opening pick. In case you‘re having trouble you can have a peek at the opening procedure of the Xperia Z4.