Ensure that the valve and cartridge each have the thermostatic control. If the valve and cartridge aren’t thermostatic, the faucet won’t work properly, and depending on the brand, may not fit over the valve.
Expose the valve and cartridge. If the shower valve was recently fitted, a plastic cover is typically placed over it to keep construction dust and debris from getting in the valve.
Put the handle screw into the valve extender until it can't go in any further. The handle screw is part of the faucet’s assembly.
Apply a bead of silicon sealant on the faucet’s gasket, located on its backing plate.
Attach the gasket over the valve. Press around the gasket to secure the silicon sealant against the shower wall.
Attach the backing plate over the gasket. The backing plate should rest securely on the gasket.
Attach the thermostatic faucet control to the backing plate securely. Some faucet controls have a specific screw to secure the fit so the faucet keeps the water supply in a safe temperature range. If the model has a screw, insert the screw and turn the screw until it’s tight. Cover the screw with the hardware supplied by the manufacturer.