I started typing up a response trying to describe exactly
how i was doing mine then thought and realised to was too hard to explain.
Basically hold your left hand (assuming your doing your left tris) over your
right shoulder but not resting there, just hovering
From there you are basically doing whats shown in the vid but either blind
or reversed if you are in front of a mirror.
Hope this helps (couldn't really do it on myself as it would have been too
hard to film so i had to improvise a little and use a prop
).