I go with IH. Think of the way the melody interacts with the accompaniment and go from there. Like, which bits are major or minor, happy or sad. What is the song about are you telling a happy story or a sad one. Then using the melody as a starting position try and embellish using the guitar, hum what you want to play. This will help you visualize (aurolize...is that a word ?) where you want to be and then translating it into notes and scales is relatively easy.