You have to sing at least one vocal so the program has something to harmonize with.

Then I go to a MIDI track, and play the harmony notes I want along with the singing using my keyboard to send the MIDI.
I record a track of MIDI, then I go into the harmonizer and select the option to use a single track for the harmonies. The harmonizer will 'sing' along to the notes you played. I do this a few times to make the individual harmony voices.

You'll have to experiment a little with the range and male/female settings in conjunction with one another. If you play an octave too high or low it will sound silly of course. Even the portamento setting will sometimes make a difference.

BTW, this is also how you use the harmonizer to tune the vocals. Play a MIDI track of the original melody and run the vocals through it as described above.

The software version is not as good as the newer hardware boxes and you'll rarely get a 'perfect' sounding voice. To me it often sounds better when you do a bunch of voices to make a choir sound than if you use it for a single featured harmony.

I do not work here, but the benefits are still awesome
Make your sound your own!