There's basically four types of new shooters.... those that see the possibility of a slingshot, like how it can be used to cut cards, light matches and do all kinds of crazy accurate shooting, and they want to do learn to do the same, sticking with practice, learn, shoot, figure some stuff out and get better and better, all the while knowing it's possible to do some amazing things with a slingshot...
Then there's the second kind, they see something incredible and decide they'll never be able to do it, and decide never to even really try because it's just impossible in their opinion...
And the third... people who know the possibilities of the slingshot, recognize that's the top level shooters, are satisfied that they may not reach that level, but practice, shoot and have FUN anyway!
Then there's the fourth kind... an encounter that's difficult to deal with... they simply don't like slingshots and there's really nothing you can do that will convince them otherwise.
So know what you're dealing with first.... construct your instructional plan to fit the type of person you are trying to help out.
If they are the first kind, show them videos of of incredible shooters and what they're able to do... and set up incrementally more difficult challenges to do.
Second kind, get their friends involved and they may follow... focus on the fun aspects, the "big bang" of hitting large noisy targets... at greater and greater distances over the course of time.
Third kind... get a lot of interesting destructible targets, and do a lot of "stump shooting"...
Fourth kind... start with "angry birds" and build from there!