The proper arm to check your blood pressure for the most accurate results is your left arm.
When taking your blood pressure you should take your blood pressure using your left arm especially if you are right handed.
If you're left handed then you may want to use your right arm to take your blood pressure.
This is what a doctor told me when he said to take my blood pressure at home and said to use my left arm for taking the blood pressure for accurate results.
However it's a good idea to also take your blood pressure on the right arm as well to see the difference between the two arms.
Sometimes your left arm may have different blood pressure readings than your right arm does.
So it's good to check your blood pressure from both arms but always check your blood pressure on the left arm for the most accurate blood pressure reading.
Also lay still, avoid coughing, talking, moving your arm etc and keep your arm rested on something such as a table with your palm up.