This link provides succinct, practical advice about how to measure orthostatic blood pressure and how to interpret the results for orthostatic hypotension:
PRACTICAL INSTRUCTIONS FOR HOW TO MEASURE ORTHOSTATIC BLOOD PRESSURE / ORTHOSTATIC HYPOTENSION
The Emergency Nurses Association also has a good link because it provides background about possible causes of orthostatic hypotension:
EMERGENCY NURSES ASSOCIATION ABOUT HOW TO MEASURE ORTHOSTATIC BLOOD PRESSURE
That's the topic in a nutshell.