The best explanation of shooting from the standing position is taught by the AMU and CMP.
When I'm talking CMP I'm talking about Gary Anderson, Olympic Gold Metal winner. He also, some time ago, in the 70s, set the record for Standing that hasn't been broken. (300 meter ISU free rifle).
Stand 90 degrees from the target. A tad more if you have to to get a NPI but never less then 90 degrees.
Feet are "about" shoulder width apart. I say "about" comfort plays a more important part then distance.
The head is straight up. Bring the rifle to your head, not the other way around.
Bone support: No muscling. Rest your non-shooting, or support elbow on your hip bone, rib cage, or stomach, (depending on your build).
Bring the rifle to your shoulder as mentioned. Hold it firmly by the pistol grip. When I said firmly I don't mean tight. If held too tight, you can't squeeze the trigger without disturbing your sight alignment. The right or shooting elbow is left to hang naturally. Don't try to hold it up or force it down.
Now Close your eyes, and point at the target. Open your eyes to make sure you have a proper natural point of aim. You adjust your windage by moving your feet back and forth, remember to keep the body at least 90 degrees from the target. Elevation is obtained by moving your non-shooting hand back and forth on the stock. The stock rest on the non shooting hand, the hand does not grip the stock.
Again close your eyes, open them to check your natural point of aim. Keep adjusting until its perfect. Now get lined up, close your eyes and dry fire. Without moving, open your eyes and make sure you're still lined up.
Relax. Relaxation is one of the most neglected fundamentals of marksmanship.
Don't muscle the rifle. Use bone support. Don't force anything. The sling is a deterrent to good off hand shooting, it doesn't help.
Nothing helps off hand shooting like dry firing. When you dry fire, (as in shooting) call your shots and plot the calls.
Follow through it critical in offhand. To develop muscle memory for follow through, after you drop the hammer, get back on target as fast as possible as if you were going to fire a second shot. Most people want to drop the rifle as soon as they fire. What happens is you subconsciously drop the rifle as you are shooting. If you act as if you are going to fire a second shot, you develop muscle memory to bring the rifle on target. So subconcisely, you'll do that, keeping the rifle on target. Hence, good follow through.
Forget the breathing stuff you've been told, breath naturally. You'll stop when you get ready to squeeze the trigger whether you want to or not. Worrying about breathing is just some unnecessary mental problem you don't need.
I'm pretty sure I left something out. I'll think about it a bit and come back and edit this post.
I'm sure someone will disagree with this, but when that person beats Gary Anderson's record, I'll give it another look see.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071