... I simply find a complimentary note to the melody, and follow the singer's lead ... a little 'vision' and a little practice, a happy accident or two, and sometimes magic happens! Of course, sometimes, I fall flat on my face too!

There are harmony theoretical approaches that work well, if you have that training, and the guess work or 'visualization' is usually a sub-conscious attempt at the applying the harmonic theory your brain has picked up through listening and sining all those years ...