The longer end of the Madison Bar steel switch box support is inserted first along side of the metal box.

The wires are folded and the GFCI receptacle is pushed into the box.

The box is held firmly in place by the Madison Bars. It should not be loose. Note how the upper and lower screw hole tabs on the box are set back into the wall slightly. Inside the rear of the box is the bare copper ground wire attached to the metal box using a 10/32 screw.
The finished GFCI receptacle installation using Madison Bars to support the box. I usually prefer to install the box next to a wall stud so that a wood or sheet metal screw can be screwed through a hole in the box directly into the wood stud in addition to the Madison Bars. That makes the box extremely secure. However in this case the customer wanted the receptacle installed in this exact location.