Normalising spectra - pwelch method

I am trying to normalise some frequency spectra that I have created using the FFT. I was wondering how it is done with the pwelch spectra. It says that normalisation is carried out but doesnt detail how.

I have thus far being using Spectralintensities/max(Spectralintensities) But wanted to bring it in line with pwelch. I am doing this so I can effectively compare one signal to another