profk007:
- Get the 12v-5v buck converter and solder the wires from the 12v 800ma power supply to Vin and Ground-out on the converter? Would it be better to use a breadboard barrel connector so I'm not cutting up wires and potentially mixing the positive and negative side wires from the external 12v power source?
Whatever you like. Normally the wires are colour coded (at least they were in all the adaptors and 12V things I've cut up so far). Red for +, black for -. Sometimes there's some other marking on the wire showing which one is +. And of course you can just use your multimeter to confirm this.
- Use Vout (5v) from the converter to power everything, including the Arduino. Is Arduino UNO's Vin pin the same as VCC?
Markings vary between boards (and clones). Vin is usually the 6.5-12V; sometimes called RAW. If you have a pin marked 5V use that. That's for sure a 5V input.
- When grounding everything, can I utilize the ground pin on the Arduino? Since technically, the UNO will only have the 5v connected to the Vin pin and no "external ground" from anywhere else.
Ground = 0V reference (it's not necessarily connected to earth), in this case linked to the negative output of your buck converter (which in turn is probably directly connected to the negative pole of your 12V adapter).
600 mA output is very little - better get a 2A converter. Those are dirt cheap, too.