If for some reason you have the desire to decock a modern (half cock doesn't lock) 1911 on a loaded rd:
1. Point muzzle in a safe direction.
2. cover the back of the slide with your off hand wrapped around the back of the slide and your firefinger jammed between the slide and hammer.
3. Keep muzzle pointed in a safe direction.
4. Remove manual safety.
5. Put your shooting hand thumb on the hammer and simultaneously pull down on the beavertail safety with same thumb.
6. Keep muzzle pointed in a safe direction.
7. If all 6 of the above criteria are met, only now should you place finger in trigger guard and pull the trigger.
8. After lowering the hammer a fraction of an inch, remove finger from trigger.
8.5 If you pinch your finger, don't pull it away. Remove your finger from the trigger, first!
9. Only after you have removed your finger from the trigger, now you can remove your offhand from blocking the hammer.
10. Lower the hammer to the half cock notch.
11. Point muzzle in a safe direction.
12. Control hammer by pinching it between thumb and forefinger of the off hand.
13. Pull trigger and gently lower all the way down to the back of the slide.
If your thumb ever slips, the gun (in good working order) will not fire as long as either A)trigger is not pressed or B) you are blocking the hammer with your other hand. As long as you keep the hammer blocked while you're actually pressing the trigger, you will not have an ND, even if the hammer slips. All that will happen is A) hammer is intercepted by halfcock notch, or B) you pinch your finger, remove finger from trigger, then let the hammer fall to the halfcock notch.
If this seems like a lot of steps, I agree. Better to leave it cocked and locked and secured in a good holster - or chamber empty. FTR, I have practiced this many times on a 1911, but never with a loaded round. After practicing cocking the hammer (with an unloaded gun), I have come to the conclusion I have no reason to have a 1911 in that condition. (Particularly one with a protruding, target-style rear sight!)