You bought one but don't know how to use it? Didn't the fellow you purchased it from show you how before you bought it?
They're really pretty straight forward (depending on how it's configured.)
Pre-heat the dyno oil/fluid. Gear it 4:1, warm up the engine with slight load, then start making pulls @ WOT. The more output the pump makes, the higher the HP. Record the rpm and pressure at each 250 rpm interval then do the math. Watch your hydraulic fluid temp - if it increases much, your numbers will magically increase making you feel like a dyno-hero. Install a thermostat with a small fan and heat sink. Even better, and old Ford radiator that has a built in transmission cooler with a squirrel cage fan in front of it.
It's actually a lot simpler once you install a load cell and data acquisition. Making runs and trying to read two gauges at once can be difficult at best.