I keep hearing differing methods!! I’d like to know how I’m suppose to sit?

To be clear: I am taking my own blood pressure & was urged to do so & keep track on the advice of my doctor.

Which is a myth?:

• Can’t have eaten/drank anything for the last half hour
• feet flat on the floor
• lying down but sitting up
• back against the chair
• don’t cross your legs/ankles
• only use your left arm
• hand facing upward
• hand facing downward
• keep your arm down
• keep your arm raised to the level of your chest/heart

What’s the correct way???

Anything else I should know? I keep getting inconsistent results, and it’s stressing me out.

Thank you.