Arrangement is the work of assembling material such as melody, chords, and rhythm into the form the song actually takes. You decide which instruments to use, where to add or remove parts, and how the piece develops from intro to outro. With the same melody and chords, a different arrangement gives a completely different impression.
Difference from composing
Composing is mainly about producing the melody and the chord progression; arranging is designing which sounds carry them, in what order, and how they are heard. In real songwriting you move back and forth between the two, so they cannot always be cleanly separated.
What arrangement decides
Instrument and tone choices, the register of each part, rhythmic density, how many parts play in each section, and the use of sound effects all fall under arrangement. Keeping the verse restrained and thickening the chorus is an important arrangement choice too.
Start by separating roles
Decide the role of each part one at a time: bass holding the low end, drums creating rhythm, accompaniment playing the chords, and a lead or riff leaving an impression. Stacking too many parts in the same role blurs what you want heard, however many sounds there are.
Creating development
In arranging, you think about what changes and where, from the intro through the verse and chorus to the outro. Contrast, not only adding parts but deliberately resting them, swapping tones, and changing rhythms, is what gives a song flow.
How to start in your DAW
Sketching the whole song with just three parts, piano, bass, and drums, is an easy way in. After that, add the sounds each section needs one at a time. Studying finished songs to see what enters and drops out at each bar is also useful.
