A relatively simple concept that helped my a bunch in writing riffs and melodies in natural minor. Similar concepts apply to the relative major key, but I don’t think I was ever successful in writing something in major
Let’s take the key of A minor for example.
- Notes of Amin key: A, B, C, D, E, F, G
Chords (triads) in the key of A minor:
- Am, Bdim, Cmaj, Dmin, Emin, Fmaj, Gmaj
The chords in bold [Dmin and Fmaj] are the “everything goes” chords: any note of the A minor scale will sound nice/cool/acceptable over these chords, even if you sustain it or use it on a strong part of the beat.
Conversely, with all the other chords there are some notes that sound a bit dodgy, especially when you sustain them: for example, it’s pretty hard to make a sustained F note sound good when the band is playing an Am triad.
So, a simple way that your melody can outline — for example — the Am to Fmaj transition, is to not use the F note at all over the Am chord, and then use it when the F chord comes in.
As usual in music this is not a hard rule but more of a guideline. I’m sure we can find plenty examples of composers that got around these “rules”