Place your foot in the stirrup of the crossbow! Do not forget that important step.
I hold the rope handle in my left hand and pull it snug but not super-tight, and hold it there, with my hand close to my body. With my right hand, I reach down and pull the crossbow's trigger. It goes "click" and the weight of the string is transferred to the hooks of the cocking device, which I am restraining with my left hand.
Then I slowly lower my left hand, still gripping the rope handle of course, allowing the crossbow string to move forward into the uncocked position.
You might expect there to be a lot of force pulling on your rope hand when decocking your crossbow, but it's really not bad. Decocking my crossbow takes less strength than cocking it.