Where did Deuteronomy 6:9 tell Israel to write God's words?

Answer: On doorposts and gates

They were to tie God's words on their hands and foreheads and write them on the doorposts of their houses and on their gates.

“Tie them as reminders on your hands and bind them on your foreheads. Write them on the doorposts of your houses and on your gates.”— Deuteronomy 6:8-9 (BSB)