Practice? What's that? I've been able to 'make it squeal' for over thirty years with no problem. All you have to do is hold the pick as close to the pointed end as possible and let the skin of your thumb and finger come into contact with the strings. And if you pick in between the pickups (on a two pickup guitar) or around the middle pickup (on a three pickup guitar) you'll find all the squeals you want.
I never call it 'pinch harmonics'. I have my own terminology for the technique. I call it puttin' some skin into it or makin' it squeal, because that's exactly what I'm doing.
Univox Les Paul Custom copy, circa 1977. Mahogany with maple top, bolt-on maple neck, genuine double coil humbucking pickups, and black with gold hardware. A notch above the typical garage band specials of the mid-1970s.
In my opinion, the originals are {censored} compared to the Mexican made reissues. I've played a few original 1970s Strats, and I didn't like any of them. If I bought a 1970s style Strat, I'd rather pay $800 for a MIM reissue than $1600 - $2500 for an original.