Artificial harmonics with a bass accompaniment. First learn this exercise but without playing the harmonics. When you know it fluently by heart, play the melody using artificial harmonics. The thumb (P) plays the bass as usual. The I finger lightly touches the string 12 frets higher than the fretted note, and the A finger plucks this. Some guitarists use their pinkie (sometimes called the "C" finger from the Spanish "cuarto" meaning 4th). I never learned to do that because it meant another fingernail to care for!!!