Re: Getting straight behind the rifle using a sling?
With or without illustration, it seems that it is difficult for most shooters on their own to get the hang of slinging up. This is because what is correct is not natural. It takes a coach, observing the shooter, to assure the position is correct. When the position is correct the shooter can get a good start on muscle memory development, which is what separates good sling shooters from others.
As George63 alluded, a shooter could watch a shooter skilled in sling usage do it, but, that's still no substitute for a highly skilled coach to assist the shooter in getting it right.
Also, it's not enough to just solicit a friend who is a high power rifle competitor to provide coaching. Good coaches not only know how to do it, they know how to teach it too. I was once mentored by a Distinguished Rifleman and President's Hundred shooter and learned little. It wasn't until I attended a SAFS conducted by the USAMU that I began to actually learn how to do it.
BTW, the text which properly covers prone sling supported shooting is about 3 pages, and, without a coach in presence delivering such information it's doubtful a new shooter would get it. For those interested, go to the USAMU's web site and look for their video on the matter.