Orchestration means deciding which instrument plays which part, in which register, and in what way. It originally refers to writing for the orchestra, but the same thinking helps when you combine strings, brass, woodwinds, and synths in computer-based production.
How it relates to arranging
While arrangement is the broader idea that covers the overall structure of a song and the design of its parts, orchestration focuses on who plays which notes. Whether the whole string section plays the chord, or the cellos take the low notes while the violins take the high ones, changes the sound.
Know the ranges and timbres
Every instrument has a range and a tone it handles well. Stacking many instruments in the low register muddies the sound, while stacking only high notes can sound thin. Keep each part within a range it can play comfortably and separate their roles.
Unison and layering
Doubling the same melody on several instruments, called unison (playing the same note together), is a classic way to make a melody sound strong. On the other hand, splitting the notes high and low or alternating between instruments creates more depth than having everyone play the same note all the time.
Putting it into practice in a DAW
Start by playing the same chord on piano, then split it into low, middle, and high parts and assign each to a different sound. Varying the velocity and the note timing slightly for each sound makes programmed parts open up more naturally.
Organize before you add more sound
The grander you want it to sound, the more important it is to decide what comes to the front instead of simply adding tracks. Keep the roles of the melody, bass, and rhythm audible, and add only the sounds you need.
