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

With one Madison Bar steel switch box support pinched tightly, the other side of the metal electrical box protrudes slightly from the wall.
The wires are folded and the GFCI receptacle is pushed into the box.