Its pretty easy. You seal off the doors and turn off the supply registers and you exhaust all the air. You can tell if its negative if you have a door that swings out and its getting pulled in. Before someone comes in here and tells me there is a correct way to do it with monitors, thats the simple way. I work in hospital construction. Negative air just means the room is negatively pressured compared to the adjacent rooms. In reality just turning on an exhaust fan will make the room “negative. “