Use a blood pressure cuff and stethoscope or an automatic blood pressure monitor
Wrap the cuff snugly around the upper arm at heart level
Inflate the cuff until blood flow is temporarily stopped
Slowly deflate the cuff while listening or monitoring for readings
Record the systolic pressure when the first pulse sound appears
Record the diastolic pressure when the pulse sound disappears
Write the result as systolic/diastolic mmHg
Example: 120/80 mmHg
Take multiple readings and average them if needed