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"

so as to keep all the words of this law and these statutes, to observe them

These two phrases mean basically the same thing and emphasize that the king must obey all of God's law. (See: rc://en/ta/man/translate/figs-parallelism)