Hello!
The blood pressure is supposed to be stable at all times to prevent strokes or other deadly effects. You measure the blood pressure with a sphygmomanometer around the arm and listen to the pulse. When completed, you would write the blood pressure like so: 120 over 80. If you write it any other way, it would be incorrect, so you measure the first sound first, then when the sound stopped and remember to write "over."
I hope this helped you!
I am, yours most sincerely,
SuperHelperThingy