Videos:

Sling: Primitive Technology  A great video of start to finish of a cordage sling. Demonstrates throwing...and isn't afraid to show himself failing!

Shepherd's Sling: Amazing Accuracy with Primitive Rock Ammunition:  Wanabe Bushcrafter:  Here we see what can be achieved with a year of dedicated practice. Some of the best parts of the video are the hints he shares from his experience, including sling design and length, ammunition selection and throwing techniques. 

Articles:

Sling for Sport and Survival:  Cliff Savage