Assuming that the person taking the blood pressure knows how to properly place a sphygmomanometer (blood pressure cuff) on a persons upper arm, and properly place the bell of the stethoscope on the inner aspect of the arm at the bend of the elbow, it is simple to check a persons blood pressure,
With the cuff and bell in place (and the stethoscope in your ears), pump the cuff up to around the 200 mark and slowly let the air out of the cuff. If you hear heartbeats immediately, you may need to go above the 200 mark. A good way to know about how high to go is by asking the person their normal BP range.
Be sure to let the air out slowly and steady, yet not so slow that the needle "jumps" on each beat. You will want to see it go slowly down with only a very slight pause (if any) on each beat.
The first STRONG beat you hear will be the systolic, or top number, reading of the BP. The last STRONG beat you hear will be the diastolic, or bottom number, of the BP reading. With the diastolic number be sure not to count past the strong beat, as you may probably hear a few faint beats after the strong reading beat. Practice is the only way to get truly accurate on reading BP.