I have a bedford, so I suppose it's the same as pap 30 size-wise. I can fit the bedford on my shoulder no problem, but I wouldn't do that and can't figure out how to make it better for me, so I'll consider the bedford as a hand-carry bag.
1) Bread-loaf under armpit method
Description: when you put the bag on your shoulder, the long side should be facing the ground. Both round sides should be facing front or back.
My words: I don't like it!! Like
absolutanne said, your arm gets extended. You'll be forced to lift up your arm to fall around the bag, instead of being able to keep your arm comfortable on the side of your body. I suppose you can kinda squish the pap under your arm a bit because it's canvas, but I wouldn't do this on my bedford.
2) Barrel on your back method
Description: the bag becomes parallel to your body. Both round sides should be facing the ground or the sky. Similar to
latinaModel but even more vertical.
My words: I don't like it!! The things in my bag will all be rolling down because of gravity. Then when I decide to take it off from my shoulder and carry it over my arm, I'll have to re-organize my things so that the weight balance out. Otherwise, the bag will become tilted on one end.
So, unless it's an absolute emergency, I wouldn't want to wear this over my shoulder.