How to Underlay in Voxengo Span

[Ma'azouk]

Hi everyone,

I want to use Voxengo Span to compare a reference track with the track I'm working on - I want to underlay the reference track with my own so I can compare them, i.e. have the two spectra displayed one under the other. 

There seem to be a few YouTube tutorials on this but they skip far too many steps for me and I haven't found one that uses Cakewalk yet.

Is anyone able to help, or point me in the right direction please?

Thanks v much ?

M