I originally posted this in "The Hunt" at
http://www.thefiringline.com/forums/showthread.php?threadid=46918
You've got at least two choices, loop or hasty sling.
For the "loop sling" (this will give you a very steady sling, but takes a little longer):
1. Hold rifle in non-trigger-finger hand with muzzle straight up.
2. Detach the rear sling-keeper so the bottom of the sling hangs down.
3. Twist the sling a half-turn clockwise (if righty).
4. Insert arm into open loop at "bottom" of sling all the way up to the armpit. Make sure rear loop isn't too tight.
5. Reach around the "outside" of the sling and grasp the forestock just behind the front sling-keeper.
6. Tighten front sling keeper so the rifle is held firmly in place when you shoulder the weapon normally. Not too tight, but not loose, either.
Hasty sling:
1. Point weapon down-range, holding it with trigger-finger hand so sling hangs down in a loop.
2. Drop left (non-trigger-finger) arm inside of loop, then bend arm at elbow so hand comes out other side of sling.
3. Twist the sling a half-turn clockwise (if righty). If you can't figure out the "half-turn" part, just make sure the sling lays flat against your arm.
4. Tuck sling all the way up to the armpit. Make sure loop isn't too tight.
5. Reach around the "outside" of the sling and grasp the forestock just behind the front sling-keeper. The sling is now wrapped once around the arm, touching at the armpit and across the bicep, then around the outside of the wrist.
6. Tighten sling keeper so the rifle is held firmly in place when you shoulder the weapon normally. Not too tight, but not loose, either.
If you have a leather National Match type sling, there's great illustrations at
http://www.fulton-armory.com/FAQ.htm#M1