Press a small board against the bottom of the outlet box to hold it flush with the ceiling while attaching the support assembly to the joists. Working through the channel, drive deck screws or nails through the holes in the metal hangers and into the joist. Use four screws on both ends of the support, two on each side of the hangers.
If you want to use a source with the maximum wires already present, it may be possible to split the source before it enters the box. Cut access to the wiring through the wall material and add a junction box to splice the new outlet into the circuit. Cover the box with a blank plate to protect the splice.
If there are several ground wires, tie them all together with a short pieces of wire to connect to the terminal screws. There is no hole in the receptacle or switch for the ground wire, only a screw.
Hold the wires together so the ends match up and twist them together in a clockwise direction using lineman pliers. Make 1 or 2 turns to form a solid union being careful not to dig into the copper with the plier jaws.