The way he is putting that shirt on is for one that is already broken in. You don't want to pull the collar down when breaking in the shirt. Also pay attention to the sean on the sleeve. It is not always on the bottom of the arm since a Katana has twisted seams. Depending on the length of the sleeve it will twist around to the side. Just put it on straight and note wear the seam ends up. You can "tweek" it up or down from there.
I put my shirts on and seat them pretty much by myself. Sometimes I need help getting the back pulled down since I can't reach that part. LOL I start with the shirt "inside out". Put my arms in (with the sleeve seam where I want it), then pull the shirt over my head. It ends up "rightside out". I seat the armpits by pushing a barbell into the armpits on the bench or squat rack.
I've found that having someone else seat the shirt makes it inconsistant. When I do it myself, I know where I'm at. This may not make to much sense. It's much easier to explain in person. I would highly recommend that if you're new to shirted benching to go to a seminar.
"I can't wait 'til tomorrow, 'cause I get stronger every day!"