I apologize to you Great War purists for the Garand images below. The clearest illustrations I found for the use of the M-1907 sling came from a 1951 U.S. Army manual

FIGURE 2. Arrangement of sling through swivels

1. Thread the feed end of the long strap through the upper keeper, then place the upper hook in the third or fourth pair of holes near the feed end of the long strap. Engage the lower hook in the pair of holes below the upper hook. The sling is now attached to the rifle.

FIGURE 3. Attaching the sling to the rifle

2. To tighten the sling, grasp the inside strap of the sling near the trigger housing with the left hand. With the right hand, grasp the sling between the hooks. Now pull toward the butt with the left hand and push toward the muzzle with the right hand until the sling is tight. Slide the lower keeper toward the muzzle until the feed end of the long strap has been passed.

FIGURE 4. Tightening the sling

3. To loosen the sling for carrying purposes, slide the lower keeper down from the feed end of the long strap and grasp the inside strap with the left hand. Now force the inside strap toward the muzzle and at the same time pull the outside strap toward the butt of the rifle.

4. To hold the sling in a tight position, force the upper keeper against the stock ferrule swivel and slide the lower keeper up until it has passed the feed end of the long strap.