Here are the tools: A Stethoscope and a Sphygmomanometer.
Make sure you place the round end of the stethoscope 'under' the blood pressure cuff.
You inflate the sphygmomanometer (blood pressure cuff) to a little above 180 mm Hg.
This collapses the major arteries to the arm (that's why it is uncomfortable).
Then you slowly release air by gently turning the air valve, and watch the pressure drop.
When you first hear a sound, that will be the Systolic blood pressure.
The sound you hear is the blood now flowing in the artery of the arm.
This means that the systolic pressure is now greater than the pressure in the blood pressure cuff.
As you continue to watch the pressure drop, when you no longer hear any sounds, that will
be the Diastolic blood pressure.
Now let's try it. With the blood pressure cuff deflated and the air valve closed,
wrap the cuff around the patient's arm as shown.
Pump up the cuff until the pressure reads about 180 mm Hg.
Then slowly turn the knob on the air valve to release air from the cuff.
As the pressure slowly drops, listen for sound with the stethoscope.
When you hear the first sound, note the pressure on the dial, that is the Systolic blood pressure.
When the sound disappears, that pressure will be the Diastolic blood pressure.
While you're at it, how about measuring the heart-rate.
Count the number of pulses for 60 seconds, and this will be the heart-rate in beats-per-minute.
Or just count the number of pulses for 30 seconds, and multiply that number by 2 for H.R. in bpm.