Using the Rifle Sling for Support

Also, some articles:

• "How to Use a Rifle Sling for Accuracy and Comfort"--photographs and article describing a hasty sling method for hunting/field use--and "How to Use a Rifle Sling," both by Russ Chastain.
• "How to Use a Rifle Sling" at Lucky Gunner--an article that briefly covers using different types of slings. 
• "Rifle Shooting Basics: The Long-Forgotten Loop Sling" by Peter Lessler.
• "How to Use the Sling, Part 1: The Shooting Sling to Arm Connection" at Art of the Rifle.
• "How to Use the Sling, Part 2: The Support Hand" at Art of the Rifle.
• "How to Use the Sling, Part 3: The Half Twist" at Art of the Rifle.
• "How to Use the Sling, Part 4: Getting it Tense (but not two tents)" at Art of the Rifle.
• "The Loop Sling" at Art of the Rifle.
• "The Secrets To Using A Traditional Rifle Sling To Its Full Potential"--an article looking at the USGI -style web strap.
• "How to Use and Adjust the Sling For Target Rifle Shooting" from CMP--how to use and adjust the loop sling.
• "Rifle Sling – What The Pros Use" at Precision Rifle Blog--a survey of what the top competition shooters are using.

The AK sling is much more basic, but here is a video discussing its use:

And a WWII training video that, at the beginning, discusses how to use the sling.