There are two things to do before you start.
Check your machine against the one in your doctor's office.
Second, make sure you have the right size cuff—the inflatable part should encircle at least 80% of your upper arm.
To get the most accurate blood pressure reading, support your arm at heart level, wrap the cuff around your bare upper arm, and follow the directions on your machine.
It's important that you not exercise (even climbing a flight of stairs can elevate blood pressure temporarily), eat, drink caffeine, smoke or use alcohol and drugs within 30 minutes of your reading.
When you first start to check your blood pressure at home, measure it early in the morning, before you have taken your blood pressure pills, and again in the evening, every day for a week. After that, follow the plan your doctor recommends, or check at least twice a week.