If you have a blood pressure measuring device, follow the directions.

To ensure it is accurate, take it to a medical professional and ask them if they will help to verify the accurracy or what they advise. One person I know did this and the nurse they saw took it a few times, had them run the machine a few times, and told them how to position themselves. They also were able to confirm it was a couple digits off and gave them the ranges they should worry about when using the machine.

The nurse also noted that a precise value is not super important, just whether it is outside a safe range or has large changes in a short period of time because blood pressure changes constantly. Taking it the same way each time is the most important thing to help notice changes over time.