What swampman described is called a 'hasty sling' its used mainly for taking offhand shots from standing when there is little time to 'sling up.'

Police and highpower shooters use a cuf sling which attaches to the upper arm and locks the rifle into place.

Here's a relatively crude set of pictures on how to use a 1907 sling. http://www.fulton-armory.com/slinguse.htm

On using a web sling http://iowahighpower.com/dsm/downloads/websling.PDF#search='using%20m1%20sling'

There's also a sling out there that is being used more and more by police tactical shooters called the 'ching sling.' This sling has some of the advantages of a cuff sling like the 1907 and M1 slings, but can be put into use about as fast as a hasty sling with much more support.

http://www.thewilderness.com/pdf/Chingsling.pdf#search='ching%20sling'