# When he sits on the throne of his kingdom

"When he becomes king" 

# he must write for himself in a scroll a copy of this law

"he must personally write in a scroll a copy of this law for himself" or "he must appoint someone to write in a scroll a copy of this law for him"

# from the law that is before the priests, who are Levites

"from the copy of the law that the Levitical priests keep"