torsional vibration: how to generate lumped parameter model?

Hi all, I am trying to learn Holzer method to do a quick estimation on the torsional natural frequency of the compressor train (motor, gearbox & dry screw compressor). I am unsure how to generate the lumped parameter model itself from the compressor train, e.g. treatment of shaft inertia, rotor stiffness etc. Appreciate if anyone can point to good resource / has good advice on how to generate the equivalent inertia and stiffness moel.