Step 2: Build Control Box
Take your electrical box, and cut out the openings for the Old Work receptacle boxes on each side Be sure to check for room for all of the components (relay, wall wart, and wires) in addition to the switch covers in relation to the lid when laying out where to cut these openings!!!
Drill holes for the 110v AC power in and the 12v DC out. Be sure to try and closely match your holes for the wire to your wire size. I removed a power connection from an old/dead PC power supply by opening the power supply and unscrewing / desoldering it and installing it into the main control box so I could use PC Power supply power cords for my AC in connection.
Be SURE you are routing the 12v DC through the floor pad and NOT the 110v AC! We don't want to see you electrocute your pooch after all. :)
If adding the optional power/light switches, you'll need to also wire those up (some knowledge of how to wire these up will be required and is an advanced configuration)
Solder your 110v AC in up to your relay. A schematic should be imprinted on the relay itself, but if in doubt break out the multimeter with continuity tester and verify / identify what each of the 8 terminals are. Most likely you will want to connect to third row of pins. You can test your relay also by connecting the 12v DC wire ends / leads to the relay's vertical terminals. You should see the relay make the connection/disconnect as you apply / remove the 12v DC.