When you measure blood pressure at home, you have to follow a few important rules.
First you need to have the appropriate instrument (a list of these is available at: http://www.dableducational.org/sphygmomanometers.html) which has been calibrated (older device can be less accurate) with the batteries charged (a drop in battery charge can affect the measurement), and a cuff of proportional size to the circumference of your arm (ask the vendor for an appropriately-sized cuff, particularly if you are severely obese, or have very muscular arms).
Then, you need to sit yourself down in a quiet place, position the cuff at the height of the heart, and take the measurement, taking factors that could make your blood pressure rise above the average, like cigarette smoking, intake of coffee, tea, energy drinks or other irritants, physical exertion, acute emotional stress (e.g . after a heated discussion or argument) into account.
The measurement should then be repeated at least 2 times (3 if the first is clearly higher than the second) at intervals of 1-2 minutes, and the average of the 3 values obtained taken (again, excluding the first where 3 values are measured).
Thereafter, measurements should be taken using the same arm, at comparable times.
It is useful to keep a blood pressure diary (See example file: Blood Pressure Log), and to take repeat measurements at shorter time intervals, especially when trying to understand whether there is actually hypertension, or when starting anti-hypertensive therapy, or occasionally when changing the latter in the context of stabilizing a treatment.