We’re going to harmonize an existing melody. Let’s take the tune Greensleeves (aka What Child is This?). This can be played on an electric guitar as it’s fairly high on the neck.
Here are the chords and melody in the key of Am. The melody moves around a bit on the neck so it will accommodate the notes we add later. Note that some measures have 3 quarter notes and some have dotted quarter/eighth/quarter. We can vary this rhythm as desired, since it might be harder to do with added notes being played at the same time, which we’ll do next.
Some chords last for two measures which is quite long so we will add a few chords to spice things up. One common technique is to replace a major chord with it’s relative minor and vice versa. So for Am we can add the C major chord (relative major). And for G we can add Em (relative minor) to the second measure instead of playing the same chord twice.
In measure 6 and 7 instead of Am, we use F and Dm. The F is used to approach the E major chord and we use the Dm as well since it is the relative minor of F.
The first stage is to add bass notes to the first beat of each measure based on the current chord as in the first 8 or so bars. Then we add more notes to connect these notes together We also add some counterpoint such as in measures 14 and 19. Later on in measures 18 we add more bass notes (these are just chord tones of the current chord, sometimes 3rds or fifths). Here’s the result:
If you like this check out my Baroque Improvisation course.
We could take this further if we want to. Try doing this in another key for example somewhere else on the neck. As you play the melody think of the chords and the moving bass lines. We can also add a 3rd note to the chord.