The second best ever explains it best in my opinion...
https://youtu.be/VRalbzmKIEM
I remember having a lot of trouble with my line when I started playing cricket a year ago, no matter what I did most of my balls landed outside off stump which is bloody useless when you're turning it away.
I watched that MacGill video you linked to and the bit about the back foot being parallel to the crease made all the difference, it was literally the only change I made to my bowling and suddenly everything was landing on middle, middle and off and forcing the batsman to play. It's crazy how the tiniest changes can make such a massive difference.
I've been trying to bowl a wrong un, for over 30yrs, very rare I'm brave enough to bowl it in a game
But it's hugely satisfying to bowl it and take a wicket with it
I still can't bowl a googly reliably over 22 yards, I tried a couple the last time I played a few weeks back and they just came out as leggies with added top spin. My stock ball has a lot of side spin and not much top-spin so I think I'm better off trying to work out how to bowl a slider.
I've messed about with some left-arm wrist spin into the side netting from ten yards away and I actually find it easier to bowl left-handed than I do with my right hand so who knows, maybe that'll be the way to go