You know it looks easy when they describe it on Park Tool and when you see these videos, but it sure doesn't seem very easy when you are trying it yourself. I've tried to both go through the procedures step by step in excruciating detail, and I've tried to just jump in and adjust here and there. Neither has worked very well for me.
When it becomes difficult or even impossible to adjust the derailleur so that it shifts reliably and crisply, that means that something else is wrong. The derailleur hanger may be bent, the cable may be frayed, or, more likely, the cable housing is gummed up and needs to be replaced. On my first mtb, I recall spending hours taking the derailleur apart, cleaning it, reassembling it on the bike, and then adjusting it over and over again, only to wind up with the same crappy shifting behavior as when I started. It was very frustrating.
I ride in dry and dusty conditions and fine dust works itself into every nook and cranny, including into the derailleur housing. It turned out that simply replacing the housing was sufficient for being able to adjust the derailleur so that the bike would shift well again.