Jim's right.
However, for a very basic lesson in sling use go here:
http://www.seidata.com/~adking/use_of_the_rifle_sling.htmIMO a sling is of little use in offhand but fairly helpful in kneeling and very helpful in sitting or prone. Given an hour or so of face-to-face time, I would expect to be able to demonstrate and explain basic sling use to a beginner -- target-cuff style, military leather style, military web version, various 'speed' slings, the 'Ching' variant, the hasty sling, etc., etc.
Trying to explain it on a forum like this would not only be counter to the forum purpose, it would wear out my keyboard!
You need to go back to the match and follow the lead of the more successful folks there. (What you're describing is unlike any match with which I'm familiar. We shoot all offhand at steel animal silhouettes at 200, 300, 385 and 500 meters. ATC types shoot paper bullseye targets at 200 yd offhand, 200 yd sitting rapid, 300 yd prone rapid and 600 yd prone slowfire. There are lots of 'make-do' matches set up to accommodate range limitations and many, many local match formats but I know of nothing like the one you're describing.)