NEMA 6-20 does not use neutral at all.
You're welcome to use 12/3 cable, but you'll be capping off the neutral and leaving it in the back of the box. That can be useful if you ever convert it to other uses, such as a Multi-Wire Branch Circuit (MWBC) supporting both 120V and 240V loads. Yeah, you can do that.
White or gray = neutral. Green or yellow/green = ground. Others = hot.
If you use 12/2 you must mark the white wire a hot color with tape, paint or shrink tube. The ideal aesthetic choice is red. Are you caught up now? :)
There are several ways the receptacle can pick up ground off the metal box - a hard flush clean metal-on-metal contact with the box will suffice, or an outlet labeled "Self-Grounding" will pick it up off the mounting screws. The telescoping box may affect this; read instructions. Or you can add a ground jumper to a proper ground terminal on the steel box.
The two hot terminals on breaker and receptacle go to black and "red" wires. Easy as that.
Remember if you are in NEC 2020, and if your state hasn't deleted this requirement, you need GFCI protection at the receptacle if it is in a garage or basement, outdoors, anywhere near a sink, or several other places. The only way to protect a NEMA 6-20 receptacle is use a GFCI breaker. In that case the GFCI's curly pigtail neutral must be wired to the neutral bar in the panel, but the neutral terminal on the GFCI is ignored.
I’ve also seen a 12/3 diagram that shows the two hot’s to the X/Y spades and the white/neutral wire bonded to the ground wire before connecting to the Ground lug on the outlet.
No, that's wrong. I could see possibly grounding the neutral on the supply end (heck, most panels the neutral bar and ground bar are the same bar)... but on the outlet end it MUST be capped off. Don't leave it bare; neutrals are sometimes hot, hence the insulation.
Grounds are green, yellow-green or bare. Nothing else is ever, ever, ever grounded, except for the Neutral-Ground Equipotential Bond in the main panel. (which sometimes takes the form of the whole neutral bar).