What did God tell Jeremiah to make and wear on his neck as a sign of Babylon's rule?

Answer: A yoke

Jeremiah wore a yoke of straps to show that the nations must put their necks under the yoke of Nebuchadnezzar king of Babylon.

“This is what the LORD said to me: “Make for yourself a yoke out of leather straps and put it on your neck.”— Jeremiah 27:2 (BSB)