Screw sequence for Wood Subfloor to Metal Joists
I suspect that you're asking about interval (spacing distance) rather than sequence (order of installation). Sequence usually doesn't matter at all. Interval is somewhat subjective since this is a barn and not a dining room, so minor noise isn't really a concern. Here are my suggestions based on experience and not from any particular specification:


Minimum: 3 per joist - just enough to hold things down well
Maximum: Every 12" on the ends (5 screws) and 16" in the field (4 screws) - plenty to prevent virtually any movement


Consider using construction adhesive, which will really lock things together. I think it's overkill in a barn, though. 

Yes, sheets should be staggered, ideally lapping two joists. Butt ends should never align if possible. This is to improve overall rigidity since the tongue-and-groove connection will offer support to adjacent sheets at the butt joints.