I am a person with multiple interests, and this is my latest one. My "Reader's Digest" summary is that the instructions—which compare slinging to throwing—are completely accurate (definitely watch the available videos). Once you get that first loud "crack" and see the projectile soar effortlessly like a rocket, you will have an "ah-ha!" moment. From there, just try to replicate that exact motion.
After slinging a few hundred projectiles, I began to understand the wear areas of the sling and why it was constructed the way it is. There is definitely some great observational engineering at play here. Personally, I found that a heavier object was much easier to start with. Fortunately, when my sling arrived, several nearby walnut trees were dropping green-husked walnuts, which make excellent practice ammunition.
Just a word of warning: make sure to warm up before your first slinging session and don't overdo it, or Advil will quickly become your best friend! I have only owned my sling for a couple of weeks, so my accuracy is still a bit sketchy, but I am honing it with practice. It is incredibly well-built, and replacement wear items are readily available from Pan Sling. Highly recommended!