Arcos, That's a nice video and like you've commented, if you're seeing a box made for the first time, it takes a little to work out.
One word of caution as he fitted the square into the chuck, he extended the jaw carriers out of the body, this can displace a carrier/jaw, as they extend at different lengths on the scroll plate or break the scroll plate thread if out to far or at the limit it's of travel.
You can just as easily turn down that first stage between centers and size the tenions to fit your chuck jaws, then continue from there.
Cheers David