What instructions should I give to an untrained passenger for Hand propping Cessna 172N as a pilot?
That is crazy.  DON'T just rely on an person who's new to airplanes and only training is 5 minutes of showing them what to do, who may or may not react correctly when it springs to life, as the only thing preventing the plane from heading off somewhere while you try to dive clear.  Don't. Do. It.

Tie the tail. To something.  Anything.  Use the passenger and show them what to do and give them a careful briefing, but tie the tail down anyway.  You don't know how the passenger is going to react if things go off kilter.  And if tying the tail is simply not an option, and you have no choice because you have to leave because zombies are approaching, at least point the airplane toward some obstacle just beyond the normal turning radius, like a wire fence, for it to run into when it takes off with your passenger confused and frightened because they pushed the throttle in too much and let off the brakes when the thing jerked forward on them, with you diving for your life.

Once it's running at idle and everything is kosher, set the parking brake and you can walk back to untie it with your passenger ready to switch the ignition off if it starts to roll away.

And make sure to use the correct terminology with your assistant.  Don't say "switch on/switch off".  It's "Switch off" for ignition off, and "Contact" for ignition on (it's old fashioned sounding, but it works).

I have been personally burned by a guy who did things the usual way, when I said "switch off", then repeated "switch off" to make sure he heard it.  He thought I'd said "Switch off" then "Switch on" when I'd just said switch off twice.  I neglected to brief him on the use of the word "contact" so it was my fault in the end.  Anyway, I got a nice noisy surprise when I turned the prop over thinking ignition was off.

Hand propping is quite a dangerous operation, especially on tri-gear airplanes where you have to lean forward to do it.  I trust you yourself have been properly trained, and know things like not to hook your fingers over the blade so you don't get pulled into it if it kicks back, that sort of thing.