I always find it funny to watch videos of people doing this with the keg upright. Many people use a screwdriver and a rag.
I prefer to take a scrap piece of wood and drive a drywall screw into it, leaving about 1 inch exposed above the wood. I turn the keg over and put the ball valve right on the screw and push down. The upside down part works best because any beer in the keg doesn't also come rushing out the dip tube. The beer (often stale and nasty) stays in the keg.
If there is a fair amount of beer in the keg still, you may need to degas more than once depending on how quick you are at dismantling the valve/spear. If you take to long the beer will be de-carbing and re-pressurize the keg somewhat.