I have written only a very few instrumentals. As I think about them, they have come about through two approaches: 1) A particular "chord lick" on which I happened to "stumble on" while knocking on a guitar, bass, or keyboard; 2) a novel use of some electronic or novel recording technique. Of the second class, I recorded one song on which I featured "reverse playback" of a four-bar melody line on the lead guitar. That took a bit of doing because I had to first play that melody section "backward" on my guitar, then use an old half-track mono recorder to play the track backward so that the guitar attacks occurred at the ends of the notes and they faded in for their beginnings. In another case, I used a Motorola HEP-583 JK Flipflop to divide the pitch of my guitar by two to get a very crude likeness to the commercial devices such as the Polyphonic Octave Generator. My version was quite flaky and unreliable; but with practice, I got fairly good at playing just heavy enough to get reasonable duration on the note and its octave below. Interestingly, if I played a perfect fifth on the guitar, it generated a "resultant 32-foot" (borrowing from pipe-organ terminology) giving a pitch an octave below the lowest pitch from the system. That function was also unreliable, but it happened often enough for me to notice it. I won't try to explain here the interaction of pitches in a nonlinear system such as passing the notes through a digital flipflop, but the process is something like this: If you feed two signals at a frequency relationship of 3 to 2 into a purely digital circuit such as a flipflop, their interaction wil generate the "difference frequency" of "1." Thus if you feed 200 and 300 Htz into something like a flipflop (a pure perfect fifth is i the relationship of 3 to 2), you will also get the difference frequency of 100 Hz.
Yes, it is at times interesting what can happen when one is playing with a bit of knowledge and a junkbox full of spare parts!