Not sure why you want to go into such detail about moving a tractor unless it's critical to the story line, but seeing as you asked:
Couple of pretty good posts here, even the tongue-in-cheek ones are pretty much on target and informative. Something I haven't seen: If it's cold out, he'd need to hit the glow plugs for several seconds before starting the tractor. Assuming the 850 is the same as my 750 was, that would mean turning the key to the left and holding it for a 10 count or so before turning it to the right to kick the starter in.
Trans in neutral and clutch down is important. Differential in neutral is unnecessary. May need to ensure the PTO is out of gear too, there's another safety interlock associated with that.