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

With one Madison Bar steel switch box support pinched tightly, the other side of the metal electrical box protrudes slightly from the wall.

This photo shows the 12/2 NM-B cable brought into the metal box and clamped. The ground wire is attached to the box using a separate 10-32 ground screw. Consult Article 314 in the National Electrical Code for the correct size electrical box needed for the amount of wires and devices that will be contained within.
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.