Most of us old enough to have used them have forgotten how!!! Trust me, for target shooting there is no advantage, there is considerably less support otherwise you'd see them in World Cups and the Olympics. The only advantage is to double as a carrying sling.
If you really want to know, hold the rifle muzzle up, put arm elbow first through the hanging loop, then put hand outside the loop around the front of the sling and circle it back to the stock fore end.
Good shooting
Robin