Unscrew the front of the device.
Use a stencil to mark the spot on the wall for the holes. Then drill the holes and place the dowels in them.
Screw the device to the wall and attach the gaskets.
If you intend to install your own cable in the device, remove the power cable and connect according to the markings on the PCB your own cable.
Then screw on the front and start the power supply.