Well...

Vary the load to hit each required RPM set point, and record.

You should be aware that a rope break dyno is going to give you, at best, a rough estimate of output. It is very difficult to get accurate, repeatable numbers with such a system.

Of course, with a genuine classical rope brake, you need to take your data quickly, and detension the rope before it burns.

You react the rope brake's torque against a scale, record the scale's reading, and also record the engine rpm at the same instant. You compute torque from the brake's arm length and the scale reading, and you compute HP from the torque and RPM.

The first thing you might try to acquire or make is a water brake (Froude) dynamometer, or an eddy current dynamometer, either of which have effective cooling so you can take data at a slower pace.

The instrumentation that comes with commercial water brake dynos will measure torque and rpm simultaneously, and plot torque (and HP) vs. rpm in real time as you ramp the throttle open and control the dyno to find peak torque at each speed.

Speed shops around here can be heard making dyno pulls that last just a few seconds, which is probably a good way to tune an engine for drag racing.

... but that's not how SAE does it.

At sae.org you will find extensive instructions for gathering engine data that meets standards for publishing and comparison, but a rope brake won't do it, because you need to let the oil temperature stabilize at each data point, etc.
Join SAE for a modest discount on tech pubs.

Mike Halloran
Pembroke Pines, FL, USA