How to quickly harmonies to many parts, melodies or voices

1st thing we must know is bass.


2nd thing we must know is the code (or numbers) above the bass. This defines the chord [Bach].


Then as the main melody flows, the other parts can be the other notes of the chord. It can sound brilliant. A possible example is the intro of I have a Dream - ABBA. It can sound rediculous if other parts just follow up or down the main melody. More than often the less movement a part has the more refined or sophisticated  is the end result, to listener's brain. The listener's brain perception. Later addition: the first and second melody can go together up or down usually in thirds but again the chord structure should be followed in order to avoid nasty surprises.


Now an example of brilliant lazy or standing parts. Listen or play Dancing Queen - ABBA. When they sing "Dancing queen feel the beat from the tambourine...", the bass stays A, pedal bass. The harmony is A C E G to A C E A then A D F G to A D F A. Notice the principle of least change in refined music. Note how A is pedal on an upper part too.  (To play correctly notice that the key is A major so all C, F, G are sharp). This sound brilliant also on a synthesizer and even better if the Pleiades tuning is used. For example on DX7ii or Korg Triton were octaves are tunes for current octave brain perception.


See also the principle of laziness in music - euroelectron


Reference:


https://normanschmidt.net/scores/bachjs-general_bass_rules.pdf