Unhook the snap from the butt, and open up a loop on that end that will self-tighten on your arm. Turn it 1/2 turn to the left and get the loop up high on your bicep, pull it snug. When you have the sling adjusted so you have to push the butt away from you a bit to get the butt into the pocket of your shoulder it will provide enough tension to keep the loop where you put it. I find that in my case when the end of the tab comes back to just about the trigger location the length is about right for sitting and prone. Fiddle with it a while and you will get it.
The M1907 sling is very good but more complicated in use. Some of us use it with the claws to the inside (against the rifle stock) which puts scratches on the finish) but is much easier to tighten and loosen and get in and out of. I think Turner sling or Fulton Armory has instructions on the leather sling--check that website.
In the old days quite a few highpower shooters used the web slings and were content with them. They could and would stretch, especially in the wet, but then so does leather to some extent. I like leather just for tradition and the feel, but the web gets the job done just about as well.