Always start with tennis balls or something other, my favourite are pine cones as they fly good and are very abundant here, more than stones! and easy to comprehend technique like the underhand, most people seem to get that one fast and then after they understand what to do and what not to do let them try(still not with stones) another technique like helicopter, balearic, byzantine, "greek"
(notice how I didn't mention figure eight, well most beginners don't really understand how they should do it as it looks confusing to them when more proficient slingers do it and try to explain it. It usually ends with them whacking themselves with the pouch with pine cone)
This is my experience and how I do it.
Or try giving them a staff sling, much easier to use and about as addictive as a sling