the best training for squats is to squat.
with the guy doing the knees together vertical shins squat...
his shoulders were fairly slumped forwards (shifting his center of gravity forwards). works better for guys who have a slightly higher center of gravity (due to relative upper body mass) rather than women who have a slightly lower center of gravity (due to relative lower body mass).
he also tucked his tail under (the infamous 'butt wink' that you need to avoid in weighted versions of squats).
how to train for it???
i read that one starts out with ones heels supported by a telephone book. then every day one removes a few pages. gradually... over a relatively long period of time... by removing a few pages per day... one can squat with barefoot in this fashion.
things to note:
it is a very cool party trick but it is a different movement from weighted squats (which are probably a priority for you). in particular: that version involves forwards slumping of the shoulders (not good for weighted versions) and also loss of lumbar arch (not good for the lower back under load).
i'd be really interesed to see vids of your squats before making suggestions on how you can improve them.
in particular: side on angle to assess knee travel forwards / lumbar arch / depth / balance on heels / torso angle...
and front on angle to assess stance width and knee travel outwards.
there is only better an stronger from here!!!
ps. don't be afraid to post vids with just the bar. often... things don't get any better with additional weight.
my new motto: (powerlifting low bar squats aside: if you can lift a stick PERFECTLY you can lift anything at all).