Concealed shower arm & flexible hose outlets - how fit

[Rhuddleston]

Anyone know how to fit these in concealed arrangement?
The shower arm came with backnut - what use is that when you can't access the back? But it needs to be fixed sturdily to hold the shower head's weight. And if it's not tightened back then the outer chrome trim won't sit flush. Surely I don't have to exactly measure to a wall-mounted back plate and screw in after tiling, and hope it won't leak, and if so what's the back nut and washer for?
Flexible hose outlet came as pictured - trim slides forwards and backwards slightly but again it needs to be sturdy.
Thanks.

 

[ShaunCorbs]

fix and pipe up after tiling so you need access to where they going behind

 

[Destroyer2813]

Throw the backnuts away mount a backplate elbow in the wall on a solid piece of 4x2 then just wind the head into it after tiling