The following information comes from The Elusive Ten by William Reichenbach. The Elusive Ten is also available to purchase in print.
This, although important, is very simple to do. (True, you know how to breathe, after a little spanking, upon your entrance into this vale of tears. However, I am talking of conscious and measured breathing, suitable for shooting.)
Now, without exertion, inhale and exhale slowly and deeply a few times.
When you are ready for your shot, inhale normally. Let some of the air out of your lungs, and then lock your throat.
With a comfortable amount of air locked in, none of your chest and neck muscles are strained, and you can easily hold your breath (which you must do) while you take aim and fire.
Do not hold your breath so long that your hand shakes while aiming. As soon as you begin to feel that the gun is putting on weight, lay it back into your left hand and put your left thumb under the cocked hammer.
When you are again ready, fit the gun back into your gun hand (See Illustration I) and free the hammer only after the fitting is completed. That way, a cocked gun will be safe at all times.