Col. "Jeff" Cooper's "The Art of the Rifle" book would be a good place to start.
Most of the best commercial schools *are* commercial operations - hence if you go to a website for "Gunsite" or the like, they'll have a few photos on their website, but nothing that resembles any kind of online "instruction" in the fullest sense - you have to book in for one of their courses.
I put a ching sling setup on my 10-22, it works good.
My M1A milspec sling is another matter. Its just a carrying sling to me but I hear rumors that they used em' for shooting slings also. I have tried different techniques with this sling and cant get it, too uncomfortable.
Is there a tecnique for the use of this sling? Sources, Tips?
"Always place your clothes and weapons where you can find them in the dark"
The milspec sling on my government issue Garand was very effective. Later when sporterizing a couple of Mausers, I outfitted both with leather military type slings. They are quick to get in and out of for shooting and are very steady. If you are doing a lot of bullseye shooting versus hunting/plinking I imagine you could get very specialized in this area.