The system of sling use I am about to describe is the method Bob Whichard of the Virginia State Rifle team trained me with, and it seems to work well for me.

Gather the rifle and sling together and place the rifle on a flat surface in front of you with the butt towards your strong (dominant or firing) hand.

Disassemble the sling into its four components: two keepers, tail section and the main strap.

First install the tail on the rifle, grasp the square ring in your weak hand and the frog in the strong hand.  The smooth side of the leather should face you.  Thread the frog through the rear sling swivel from the butt end towards the muzzle.

Fasten the frog into the first set of adjustment holes available, yielding the longest effective tail length.

Turning now to the long strap, grasp the sling by the free end and thread this into the first keeper and with the smooth side towards the rifle, pass the free end through the front sling swivel from the butt end.  Thread the free end back through the keeper you previously installed on the sling.  Install the second keeper on the sling.,  Thread the free end through the tail rectangular ring from the smooth side of the tail insuring that the sling and tail are oriented flat in relation (not twisted) to one another and the smooth side of the sling is closest to the tail as it passes through the square ring.  Pass the free end back through the second keeper and fasten the main strap frog into its free end.

Properly assembled the sling will have a keeper on either side of the frog in the main strap.  The sling will have the metal frogs towards the rifle.

Sit on the ground with the butt of the rifle between your legs and the magazine well or floor plate facing to your support side.   Reach past the support hand side of the rifle with the firing hand grasp the sling and twist the firing side towards the rifle about one third of a turn (120 degrees).

Using both hands adjust the keepers and sling to open the loop below the lower keeper which passes through the rectangular metal ring of the tail.  Open it  large enough for your arm to go through.   Thread the support arm through this loop. Bring the loop as far up into the armpit as you can.  Attach and adjust the sling strap on the jacket if so equipped.

Rotate the  “cuff” about the upper arm as far as you can to the outside and hold it in place by squeezing the support elbow tightly to the side of your body.  Grasp the main strap only between the keepers and pull it across your body. This will cinch the “cuff down on the upper arm between the deltoid and the biceps.   Repeat the process of twisting the “cuff to the outside,  and pulling the inside strap across the body until the sling is very snug.  Now push the muzzle away from the support shoulder.  This will draw the slack through the front sling swivel to even up both sides.  The lower keeper should be wedged between the main strap frog and the upper arm.  Slide upper keeper down to trap the free end of the main strap against the base of the main strap frog.  Wrap the sling about the support forearm by making a circular sweep about the sling main strap with the support arm and grasping the forestock.

To adjust the length, loosen the sling by pulling the frog side outboard and then rotate the loop on the arm and repeat the process until the loop about the arm is large enough to permit you to pull the lower keeper off the frog.  Then slide the upper keeper up and change the frog’s engagement in the free end’s adjustment holes as required.  Then tighten the sling as above.

To remove the sling  loosen as above and slip off the jacket’s strap( if so equipped), and pull your arm from the expanded “cuff”.