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

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.

The wires are folded and the GFCI receptacle is pushed into the box.
After inserting the second Madison Bar the protruding edge of the metal electrical box needs to be pushed hard against the wall as the Madison Bar is pinched over. You can use a large flat head screwdriver for this, the bigger the easier. This ensures that the box will be held in place tightly. Use needle nose pliers to crimp the Madison Bars tightly against the metal electrical box.