What did Joshua tell the people to do, because "tomorrow the LORD will do wonders among you"?

Answer: Consecrate themselves

Before crossing the Jordan, Joshua said, "Consecrate yourselves, for tomorrow the LORD will do wonders among you" (KJV: "Sanctify yourselves").

“Then Joshua told the people, “Consecrate yourselves, for tomorrow the LORD will do wonders among you.””— Joshua 3:5 (BSB)