Panic travels faster than any plan you shout. Lower your voice and the room will lower theirs.

Groups regulate themselves by copying the most intense person present, which in a crisis is almost always whoever is giving instructions. Shout a good plan and you get a room that has registered the volume and none of the plan. Dropping your voice makes everyone quieten down in order to hear you, which is the state you needed anyway.

Slow your first sentence deliberately and make it shorter than feels natural. Give one instruction, wait, then give the next. The calm is doing as much work as the content is.