How exactly do you use a U-tube to find atmospheric pressure?

Close off one end of the 'U' and fill the tube halfway with water. The water level you're currently at is the current atmospheric pressure. When the pressure changes, it will put more or less pressure on the open side of the 'U' and the water level will change representing the change in pressure. If you know what the pressure is, you can mark the tube and 'calibrate' it.