I wrote in A citizens kit about the essential nature of a good military web sling to accurate rifle shooting, but suspect that if you are a visual learner like me, it probably went in one ear and out the other. It helps to see it done.
Luckily, an Appleseed instructors in Arizona has posted a couple of videos showing slung rifle positions. Here’s the sitting position using a “hasty” sling.
It isn’t as fast to get into, but if you have the time, I’m a huge fan of the stability of a loops sling, as the same instructor cadre shows here. If all your other fundamentals are sound, you can hit targets using a loop sling from the prone position out to (or past) 500 yards.