en_tn/deu/17/18.md

When he sits on the throne of his kingdom

Here "throne" represents the person's power and authority as king. To sit on the throne means to become king. AT: "When he becomes king" (See: rc://en/ta/man/translate/figs-metonymy)

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

Possible meanings are 1) "he must personally write in a scroll a copy of this law for himself" or 2) "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"