1. Yes, if the person has literally no idea or concept of what a squat is, a visual demonstration probably makes sense. This has it's downsides, as it's not likely to be a perfect demo, and then they'll be imperfectly copying an imperfect demo, but probably a necessary step.
2. Your last comment - control your descent. Slow down. I did an instagram post about this the other week that you may find helpful: https://instagram.com/p/7LvshsGITy/?...=wolf_strength
3. Quickly, but under control. Unless the person is doing something totally awful on the way down, save more specific instruction about how to put the bar down until after they've learned to pick it up correctly. Sometimes they're rounding so severely, or putting the bar down out in front of their feet, and you need to address this sooner. But usually if you just tell someone to put it down quickly but under control, it'll be good enough for the moment.