jasche, I think you may have just asked one of the greatest short questions that requires a long answer. Theres basically two types of slings for use in prone and sitting. I won't mess with all of the various names, but it breaks down into
1. a sling attached to the forend and the buttstock, primarily for the purpose of carying, and additionally used for support in some situations. This is the fastest sling type.
2. a sling attached to the forend, or a handstop on the forend, and your weak arm shoulder or upper arm. This sling is designed primarily for support, and has no provisions for a carry mode.
This question would most easily be answered with pictures, and hopefully somebody will provide them. I would, but my camera is toast. Ive got plenty of published stuff on sling use from Palma and smallbore, to the stuff printed in the monthly gun rags for the typical "hunting" situations. Im disinclined to post it since its copyrighted, but Id be happy to sneak it onto the scanner and into an email if nobody responds to your satisfaction.