I have tought slinging to over three thousand people so far in Museums and other events. My technique of teaching evolved over time and is now pretty ok I guess.
I fully agree with Safety first. Thus I go for greek underhand mostly. Also it allows me to teach five people at a time.
Generally I try to teach with talking and showing  in parallel
with pazses between sentences to allow people to perform the elements.

The setup is any rather large target 7-10m away., and a line for shooting.
Also I break it down in small bits.

-explain and show Functional elements of the sling + find the fingerloop* (really, there IS a loop)
-Showing the handgrip and have them do it
-Explain the release mechanism and have them release the thumb
- Have them stand 2 m stand apart on a line so that everyone is safe, show the slow rotation with the sling without projectile.
Detailed show and describe one after the other- stand sideways, only rotate with the wrist, rotate in clockwise direction, release just after the lowest point. Outline slow rotation again
The ones who manage to release at the bottom get a tennis ball.
More than 95% get the fist shot forward, and so far no one got hurt
Sometimes I feel a bit like a stewardess while explaining and showing, but this improves understanding tremendously.
Perhaps its time to make a video one day...

Oh man that is so cool.

All together now

Music strikes up.....


Here we are
Slinging on the ceiling


Woo hoo, yay   Lionel Ritchie

Morpy actually I am a Biotechnologist in the Biopharmaceutical industry.
This is only what I do at the weekends for fun to please my archaic drive

Funditor you should definitely do a video.

I always start with tennis balls but I have also graduated people to stones if they were good enough. One man had been a javelin thrower for his school so I gave him a long sling and a good stone and I swear he went beyond 200 metres on his first throw, I was amazed.