If you can locate the brachial artery on yourself and another person without blindly guessing, you're already halfway there (look at a few anatomy charts, play around with your own arm for a bit).
Put the stethoscope part on top of it, and wrap the cuff around (this will vary, depending on the design of your sphygmomanometer. It should pretty much be slightly closer to the medial side of your bicep, near the fold between your arm/forearm.
Pump that sucker up to 150 mmHg or so (make sure the pump is closed), and slowly let out air until you hear the first sound, then keep listening until you hear one final/faint thump.
For a healthy person, you should hear something around 100-130 for the first sound (systolic), and anywhere from 50-90 for the 2nd sound (diastolic).
You can easily practice on yourself, and the only way to get better is to practice though 😛