Having a well-lubricated shifter pivot and cables is a big deal, as is the use of retrofriction-type shifters, all of which allow the lowest level of force to shift, which enhances accuracy.

I think that the very best friction-shift freewheel is Shimano Uniglide, together with modern chain in the narrowest width that your chainring spacing will allow without the chain ever skating on the small ring.

9-speed chain will work best on any Uniglide or Suntour freewheel, and is almost a necessity on Ultra-spaced Suntour freewheels if you want good shifting.

Most older French and Italian freewheels won't work so well with narrow chain, and will tend to slip during shifts.

Shifting while braking and cornering is done under no power, so there may be less audible feedback as to shift progress.
As long as the cornering and braking take full mental priority over shifting, the technique is learned quickly.
And, obviously there are times when taking one hand off of the bars is not be a good idea.

Oh, and I do all my shifting right-handed, so that my dominant left hand stays on the bar.

The fit of the bicycle is also VERY important to friction shifting, the rider must remain comfortably in control during shifting.

So, a lot of " shifting technique" is really in bike setup!