The best way to learn is to experiment for yourself. There is no substitue for experience.
I agree, you should definitely read the articles section for descriptions and videos of good techniques.
The classic 'helicopter style' whirling around the head (no more than two windups) isn't very realistic for massed ranks or for hunting in thick brush, however it may have been used in open fields.
An underhand style, similar to that of a softball pitch, with no more than two windups (probably just one) is generally agreed upon as the best for distance but probably not accuracy.
The Apache style is perhaps the most accurate, it uses no windup and so also has reduced range (there is an entire article on it in the articles section)
There are many other techniques including overhand, greek, sidearm, and one very similar to a tennis serve.
Also, please be aware that slinging has a notoriously steep learning curve, so the charachter learning how to sling profficiently in one day would be highly unrealistic for a mortal.
Things you may like to know:
~It's generally agreed upon that the sling can outrange the bow.
~Sling ammo includes rocks, lead, and clay; formed ammunition is referet to as Glandes.
~Refer to
http://www.slinging.org/forum2/yabb/YaBB.cgi?action=search2 as to the amount of power sling ammo has (especially my post three or four down
)
More coming later if you're interested. Any further questions are welcome