You will notice the arm is brought around the outside of the sling, and the back of the wrist lays on top of the sling.( Standard for both sitting and prone).

You will want to try and grip the firearm with both hands, but when your position is correct you will find you can really relax your arms and maintain your position.  Any tension in your arms or hands will show up on your target.

My standard head position, as with most AR shooters, is with the nose on the charging handle.  It is SO important to maintain a consistent head position.  As a new shooter it is hard to understand.  You think "sight alignment is all that matters".  WRONG!

I like to keep my right elbow close to my body.  It seems to allow me to relax more.