I've seen this subject pop up a few times and I figured I could add my two cents.
I'm only going over the theoretical side of this subject, leaving out the rhythmic aspect of things.
-I'll start off with the prerequisite knowledge before you start experimenting and harmonizing your own melodies.
Take the C major scale. You have seven notes. C D E F G A B C - Each of these can be assigned a number as well from 1 to 8.
Each of these notes can be harmonized, meaning you can make a chord from each of these chords. The first thing to memorize is they types of chords that we can get out of each scale degree. And this is done by stacking thirds on top of the note( a third = every other note)
C chord is major = I = C E G
D chord is minor = ii = D F A
E chord is minor = iii = E G B
F chord is Major = IV = F A C
G chord is Major = V = G B D
A chord is Minor = vi = A C E
B chord is diminished = B D F
Notice that we only used notes from the c major scale. You have your starting note and then you stack thirds on top of it to create a chord.
This works in any key. The qualities of the chords are always going to be the same.
There are very common chord progressions and you can always use those as a starting point to write melodies on top. Or like we're going to do, you can create a melody and create the chords afterwards.
Now remember that we're not accounting for rhythm. So lets take a melody that contains these notes. C - E - C - B - E - F- D-
You have different options for each note. For example the first note C appears not only in the C chord. It's also found in the F and A chord. Not there are traditional rules about chord progressions, these pertain to the order of the chords. These rules come from classical music and there is definitely value in them. But I'll stay away from that for this guide
C- We could start with the chord C. Mainly because it's the one and this might help us establish the tonal center
E- We could of course go to the E chord, Other options are staying on the C chord again, maybe play an inversion, that's where the bass changes to another note from the chord.
C- Here maybe we could go to an A minor chord or an E minor chord.
so you see, the melody note doesn't necessarily have to be the root of the chord. It can be the third or the fifth.
I won't go through all the notes of our melody. I would love to hear what you would write.
A really good way to work on this is to sit at the piano and hear how each chord sounds against your melody, that way your ear makes the decision not the theory part.
The last thing I want to go over is the instrumentation. The chord can be spread out across various instruments. For example the root note can be on the bass, the melody note on the synth (obviously) and then the pads can fill in the gaps. But you can also mess around with the order. Maybe do inversions and use something other than the root note for the bass.
I hope this helps. If you have any questions please don't hesitate to ask.
