To see how it’s done, first play the brutal odd-meter riff in Ex. 1. When you’ve got it down, harmonize each note that is played on the fifth string with a pitch a third higher on the fourth string, as shown in Ex. 2. Staying diatonic to the E natural minor scale dictates that some of these thirds are major, others minor. Another demonstration of this approach is Ex. 3. First, play this riff through without the notes in parentheses. Then, add the parenthetical notes—they constitute the harmony part and, this time, the entire lick is diatonic to the E Phrygian dominant scale. As is the case with the first two examples, I actually start this riff down a fourth by playing it on the lowest strings of my 7-string, where it sounds just merciless. Then, when I add the harmony line, I move everything up a string to the key in which it’s written here.
The real challenge with this type of riff, though, is probably not so much the harmony or the odd meters as it is the picking. When you’re playing a single-note line like the first example in this lesson, it’s fairly easy to keep the alternating upstrokes and downstrokes sounding even in their timbre and attack, but when you’re playing double-stops (as in Examples 2 and 3), it becomes harder to maintain that consistency. It all goes back to working on being a good, even alternate picker. I think Al Di Meola once said you should be able to hit any note in any direction on any string with the same amount of conviction. I don’t think he was talking about this kind of music, but his statement still holds true.