Compose a lead sheet first. The lead sheet might be a simple piano and vocal arrangement of the song with guitar chords notated over the top of the music staff. If there are no vocals or guitar parts, the lead sheet may be a simple piano arrangement of the composition or a simple chord progression indicating where other instruments join in. You can refer to the lead sheet as you compose individual instrument's parts for your composition.
Compose the main melody of the composition next. Depending upon the type of song you're writing, the melody might be carried by violins, flutes or guitars. Writing the melody of the song early can be helpful because a melody will typically repeat itself throughout the song. This will give you a baseline from which you can build your harmony. If the melody is a vocal, write the lyrics and notes together on their own sheet.
Write the bass and rhythm instrument parts last. This will allow you to see the structure of the song and compose a supporting rhythm throughout, using the melody and the harmonic progressions as a guide for the bottom line of your composition.
Arrange your individual instrument sheets, beginning with the lead sheet, followed by vocal sheets, then strings, woodwinds, brass, rhythm instruments and percussion parts.